DESCRIPTION REPRODUCTION APPARATUS, REPRODUCTION METHOD, REPRODUCTION PROGRAM, AND RECORD MEDIUM Technical Field The present invention relates to a reproduction apparatus, a reproduction method, a reproduction program, and a record medium that allow a program recorded on a large capacity record medium such as a Blu-ray disc to accomplish a picture-in-picture function. Background Art In recent years, as a standard for a disc type record medium detachable from a record and reproduction apparatus, Blu-ray Disc standard has been proposed. The Blu-ray Disc standard uses a disc as a record medium that has a diameter of 12 cm and that is coated with a cover layer having a thickness of 0.1 mm, a blue-purple laser having a wavelength of 405 nm, and an objective lens having a numerical aperture of 0.85 as optical systems. The Blu-ray Disc accomplishes a maximum of 27 GB record capacity that allows more than 2 hours of a Japanese BS digital high-vision broadcast program to be recorded without deterioration of picture quality. It is expected that there will be AV (Audio/Video) signal sources that are analog signals and digital signals of for example analog television broadcast programs and digital television broadcast programs, respectively. In the Blu-ray Disc standard, sub-standards for methods of recording these broadcast programs have been established. On the other hand, as a derivative standard of the current Blu-ray Disc standard, reproduction-only record mediums on which a movie, music, or the like is pre-recoded are being developed. Although DVDs (Digital Versatile Discs) have been widespread as disc-shaped record mediums on which a movie or music is prerecorded, since reproduction-only optical discs based on the Blu-ray Disc standard feature a large capacity, high speed transfer rate, and so forth of the Blu-ray Disc standard and these discs have a record duration of two hours or longer of high-vision pictures with high picture quality, they are largely different from and superior to the existing DVDs. Hereinafter, the reproduction-only record mediums based on the Blu-ray Disc derivative standard is referred to as the BD-ROM (Blu-ray Disc-Read Only Memory) to distinguish them from the recordable Blu-ray Discs. On the other hand, in the current Blu-ray Disc standard, user interface functions such as a method of displaying a list of video contents recorded on a disc, a function that displays a cursor on the list, and a function that allows the user to select a video content that he or she wants to reproduce have not been established. These functions are accomplished by a record and reproduction apparatus that records and reproduces data onto and from a Blu-ray Disc. Thus, the layout of the content list screen varies depending on the record and reproduction apparatus that reproduces the data. Thus, since there are also difference in user interfaces, they are not always user-friendly. For the reproduction-only discs, every reproduction apparatus need to accomplish user interfaces such as a menu screen as the disc (content) creator designed. A multi-story function that displays a selection screen during reproduction of a video content and branches the main story to a sub-story depending on user's selection is generally referred to as the interactive function. To accomplish the interactive function, the disc creator needs to create a scenario that contains a reproduction sequence and branches that he or she designated, code the created scenario with a programming language, a script language, or the like, and record the program onto a disc. When the reproduction apparatus reads and executes the program, the apparatus can reproduce a video content and display a branch selection screen as the disc creator intended. To accomplish the user interfaces as the disc creator intended, a method of generating a menu screen and a branch selection screen and a method of coding a process corresponding to user's input are required. However in the Blu-ray Disc standard (Blu-ray Disc Rewritable Format Ver 1.0), these methods have not been established. Thus, to date, it is difficult to cause every reproduction apparatus to reproduce a video content recorded on a Blue-ray Disc according to a scenario that the disc creator created regardless of the maker and model of the reproduction apparatus. In addition, reproduction-only discs on which movies are pre-recorded need to have a subtitle display scheme. However, in the current Blu-ray Disc standard, the subtitle display scheme has not been established. On the other hand, however, for example in the DVD (Digital Versatile Disc) standard, the foregoing interactive functions have been accomplished. For example, while a moving picture is being reproduced from a DVD-video, a menu screen is called with for example a remote control commander. By selecting a button displayed on the menu screen, the current scene can be changed to another scene. Moreover, in the DVD standard, a subtitle display scheme has been established. As the subtitle display scheme, one of a Japanese subtitle and an English subtitle that have been prepared can be selectively displayed. In the case of the DVDs, the menu screen is composed of fixed sub picture data. When the menu screen is called, the sub picture data is composed with moving picture data and the combined data are displayed. Japanese Patent Application Laid-Open Publication No. HEI 10-308924 describes a structure that combines sub picture data with moving picture data and records the combined data onto a recordable DVD. For the foregoing BD-ROMs, a technology that accomplishes a subtitle display and an interactive display has been proposed. In this technology, planes for a moving picture, a sub-picture (subtitle), and a menu are provided. The pictures of the three planes are combined into one picture. The combined picture is output. According to this related art, a moving picture plane for a moving picture, a subtitle plane for a subtitle, and a graphics plane for a menu screen, buttons, and so forth are layered in the order so that the moving picture plane is the bottom plane and the graphics plane is the top plane. The subtitle plane is combined with the moving picture plane. The graphics plane is combined with the combined picture. When the subtitle plane and the graphics plane are combined, the opacity can be set on a pixel-by-pixel basis. When the opacity of a pixel of one plane is set at 0, a corresponding pixel of the next lower plane is displayed through the pixel of the upper plane. The reproduction-only DB-ROMs need to have the so-called picture-in-picture function of which a small video picture is displayed in a large video picture. In the picture-in-picture function, in a multi-angle picture composed of a plurality of pictures reproduced in the same time sequence, while a main angle picture is being displayed on a main screen, a second angle picture can be displayed on a sub screen that is a small area of the main screen. When the picture-in-picture function is accomplished, it is necessary to provide a method of treating two video signals in parallel, combining pictures of these video signals, and displaying the combined picture on one screen. In addition, when the picture-in-picture function is accomplished, a picture having a size of the main screen is reduced so that the reduced picture can be displayed in the sub screen. In many cases, the reduced picture and the main picture are combined and the combined picture is displayed. Thus, it is necessary to provide a method of supplying a reduced picture to a moving picture plane and combining the reduced picture and a moving picture. In addition, it is necessary to provide a method of displaying a so-called wallpaper that is a picture of which a predetermined pattern is repeated in the background of reduced moving picture data. Disclosure of the Invention Therefore, an object of the present invention is to provide a reproduction apparatus, a reproduction method, a reproduction program, and a record medium that allow the picture-in-picture function to be accomplished with the BD-ROM. Another object of the present invention is to provide a reproduction apparatus, a reproduction method, a reproduction program, and a record medium that allow a wallpaper to be displayed in the background to moving picture data with the BD-ROM. To solve the foregoing problem, the present invention is a reproduction apparatus, comprising first storage means for storing first moving picture data reproduced from a record medium; second storage means for storing second moving picture data reproduced form the record medium; and selection means for selecting one of outputs of the first storage means and the second storage means on a predetermined area-by-area basis, wherein a display signal is generated corresponding to an output of the selection means. In addition, the present invention is a reproduction method, comprising the steps of storing first moving picture data reproduced from a record medium to first storage means; storing second moving picture data reproduced form the record medium to second storage means; and selecting one of outputs of the first storage means and the second storage means on a predetermined area-by-area basis, wherein a display signal is generated corresponding to an output of the selection step. In addition, the present invention is a reproduction program that causes a computer device to execute a reproduction method, comprising the steps of storing first moving picture data reproduced from a record medium to first storage means; storing second moving picture data reproduced form the record medium to second storage means; and selecting one of outputs of the first storage means and the second storage means on a. predetermined area-by-area basis, wherein a display signal is generated corresponding to an output of the selection step. In addition, the present invention is a computer readable record medium on which a reproduction program has been recorded, the reproduction program causing a computer device to execute a reproduction method, comprising the steps of storing first moving picture data reproduced from a record medium to first storage means; storing second moving picture data reproduced form the record medium to second storage means; and selecting one of outputs of the first storage means and the second storage means on a predetermined area-by-area basis, wherein a display signal is generated corresponding to an output of the selection step. As described above, according to the present invention, one of the outputs of the first storage means that stores the first moving picture data reproduced from the record medium and the second storage means that stores the second moving picture data reproduced from the record medium is selected on a predetermined area-by-area basis and a display signal is generated. Thus, the first moving picture data and the second moving picture data can be exclusively combined and displayed. Brief Description of Drawings Fig. 1 is a block diagram showing a typical example of the structure that displays picture data; Fig. 2 is a schematic diagram showing an example of the structure of a video plane, a presentation graphics plane, and an interactive graphics plane; Fig. 3 is a table describing the resolution and displayable colors of each plane; Fig. 4 is a functional block diagram showing an example of a structure that combines three planes; Fig. 5 is a table showing an example of input/output data of a palette; Fig. 6 is a table showing an example of a palette table stored in the palette; Fig. 7A and Fig. 7B are schematic diagrams describing a picture-in-picture; Fig. 8 is a functional block diagram showing an example of a structure that combines a second video plane, a video plane, a presentation graphics plane, and an interactive graphics plane; Fig. 9 is a schematic diagram showing an example of which a wallpaper picture is displayed with the second video plane; Fig. 10 is a schematic diagram describing a method of accomplishing a wallpaper picture display according to a second embodiment of the present invention; Fig. 11 is a schematic diagram describing a method of accomplishing a wallpaper picture display according to the second embodiment of the present invention; Fig. 12 is a functional block diagram showing an example of a structure that accomplishes the picture-in-picture with one video plane; Fig. 13A, Fig. 13B, Fig. 13C, and Fig. 13D are schematic diagrams describing selection timing of a switch; and Fig. 14A, Fig. 14B, and Fig. 14C are parts of a functional block diagram showing an example of the structure of a player-decoder according to the first embodiment of the present invention. Best Modes for Carrying out the Invention Next, an embodiment of the present invention will be described. For easy understanding, before describing an embodiment of the present invention, an outlined structure that displays video data and a method of combining a moving picture plane, a subtitle plane, and a graphics plane proposed as the BD-ROM HD (High Definition) movie mode standard will be explained. The BD-ROM HD movie mode has been proposed so as to provide a BD-ROM interactive function equivalent to a DVD-video interactive function. Fig. 1 is a schematic diagram showing a typical example of a structure that displays video data. In Fig. 1, only necessary sections are shown. A CPU (Central Processing Unit) 301 and a graphics section 303 are connected to a bus 300. A DRAM (Dynamic Random Access Memory) 302 as a work memory is connected to the CPU 301. A VRAM (Video RAM) 304 is connected to the graphics section 303. An output of the graphics section 303 is supplied to a display 310. The CPU 301 uses the DRAM 302 as a frame buffer and performs a predetermined process such as a reduction process for video data. The CPU 301 reads the processed video data from the DRAM 302 and supplies the video data to the graphics section 303 through the bus 300. The graphics section 303 sets horizontal and vertical scanning frequencies for the display 310 to decide a display resolution and has a graphics control chip that executes a rendering instruction issued from the CPU 301. Video data supplied to the graphics section 303 are written to the VRAM 304. The graphics section 303 reads video data from the VRAM 304 corresponding to predetermined horizontal and vertical scanning frequencies and supplies the video data as a digital video signal to the display 310. In other words, the VRAM 304 corresponds to a plane. Data stored in the VRAM 304 directly affect data displayed on the display 310. Next, the structure of planes in the BD-ROM HD movie mode and a method of combining these planes will be described. In the following description, the moving picture plane, the subtitle plane, and the graphics plane described in the related art section are referred to as a video plane, a presentation graphics plane, and an interactive graphics plane, respectively. Fig. 2 shows an example of the structure of a video plane 10, a presentation graphics plane 11, and an interactive graphics plane 12. The video plane 10 is the bottommost plane and used to display a picture (mainly, moving picture data) designated in a play list. The presentation graphics plane 11 is the next bottommost plane placed above the video plane 10 and is used to display subtitle data while a moving picture is being reproduced. The interactive graphics plane 12 is the topmost plane and is used to display graphics data for parts used in a GUI (Graphical User Interface) such as character data and bit map data such as buttons that compose a menu screen. One display screen is displayed by combining these three planes. The video plane 10, the presentation graphics plane 11, and the interactive graphics plane 12 can be independently displayed. The video plane 10, the presentation graphics plane 11, and the interactive graphics plane 12 each have a resolution and displayable colors as shown in Fig. 3. The video plane 10 has a resolution of 1920 pixels x 1080 lines and a data length of 16 bits per pixel. The video plane 10 also uses a YCbCr (4 : 2 : 2) system of which the ratio of brightness signal Y and color difference signals Gb and Cr is 4 : 2 : 2. In the YCbCr (4 : 2 : 2) system, each of the brightness signal Y and the color difference signals Cb and Cr has eight bits. Two horizontal bits of the color difference signals Cb and Cr compose one color. The presentation graphics plane 11 has a resolution of 1920 pixels x 1080 lines and a sampling depth of eight bits per pixel. The presentation graphics plane 11 uses a color system that has eight-bit color map addresses with a 256-color palette. The interactive graphics plane 12 has a resolution of 1920 pixels x 1080 lines and a sampling depth of eight bits per pixel. The interactive graphics plane 12 uses a color system that have eight-bit color map addresses with a 256-color palette. Instead, the video plane 10 may have resolutions of 1280 pixels x 720 lines, 720 pixels x 480 lines, and 720 pixels x 576 lines. In these cases, the presentation graphics plane 11 and the interactive graphics plane 12 have the same resolution as the video plane 10 has. In the foregoing example, the presentation graphics plane 11 and the interactive graphics plane 12 use a color system having eight-bit color map addresses with a 256-color palette. However, the present invention is not limited to such an example. The number of colors may be increased by changing the sampling depth and increasing the number of colors of the palette. When the sampling depth is 12 bits, the number of colors used with the palette becomes 4096. Likewise, YCbCr (4:4:4) and RGB (4:4:4) that have a sampling depth of 24 bits and no palette and of which each pixel has color information can be accomplished. On the interactive graphics plane 12 and the presentation graphics plane 11, alpha-blending can be performed in 256 levels so that these planes can be combined with another plane in 256 levels of opacity. The opacity of these planes can be set on a pixel-by-pixel basis. In the following description, it is assumed that the opacity a is represented in the range of 0 £ a £ 1, that opacity a = 0 represents perfect transparent, and opacity a = 1 represents perfect opaque. The presentation graphics plane 11 is used to display for example PNG (Portable Network Graphics) format video data. In addition, the interactive graphics plane 12 can be used to display PNG format video data. In the PNG format, the sampling depth of one pixel is in the range from one bit to 16 bits. When the sampling depth is eight bits or 16 bits, an alpha channel, namely opacity information (referred to as alpha data) of each pixel component can be added. When the sampling depth is eight bits, opacity can be designated in 256 levels. With opacity information of the alpha channel, alpha-blending is performed. In addition, a palette image of up to 256 colors can be used. An element (index) of a prepared palette can be designated by an index number. Video data on the presentation graphics plane 11 and the interactive graphics plane 12 are not limited to the PNG format. Video data on the presentation graphics plane 11 and the interactive graphics plane 12 may be video data compressed according to another compression encoding system such as the JPEG system, video data compressed according to the run-length compression system, or bit map data that have not been compression-encoded. Fig. 4 shows an example of a structure that combines the three planes according to the conditions shown in Fig. 2 and Fig. 3. Moving picture data of the video plane 10 are supplied to a 422/444 conversion circuit 20. The 422/444 conversion circuit 20 converts the color system of the moving picture data from YCbCr (4:2:2) into YCbCr (4 : 4 : 4). The converted moving picture data are input to a multiplication device 21. A resolution conversion circuit may be disposed between the 422/444 conversion circuit 20 and the multiplication device 21 to convert the resolution of the moving picture data into another resolution. Video data of the presentation graphics plane 11 are input to a palette 22. The palette 22 outputs RGB (4:4: 4) video data. When opacity has been designated for the video data by alpha-blending, the designated opacity al (0 s a =s 1) is output from the palette 22. Fig. 5 shows an example of input/output data of the palette 22. The palette 22 stores palette information corresponding to for example an PNG format file as a table. The palette 22 references an index number with input eight-bit pixel data as an address. Corresponding to the index number, RGB (4 : 4 : 4) data composed of eight bits each are output. In addition, the palette 22 outputs data of the alpha-channel, which represents opacity. Fig. 6 shows an example of the palette table stored in the palette 22. In the palette table, 256 color index values [0x00] to [OxFF] (where [Ox] represents hexadecimal notation) are assigned three primary color values R, G, and B composed of eight bits each and opacity a. The palette 22 references the palette table according to the input PNG format video data and outputs color data of R, G, and B (RGB data) composed of eight bits each and opacity a according to index values designated with video data on a pixel-by-pixel basis. A palette 26 that will be described later stores the similar palette table. The RGB data that have been output from the palette 22 are supplied to an RGB/YCbCr conversion circuit 30. The RGB/YCbCr conversion circuit 30 converts the RGB data into data of the luminance signal Y and the color difference signals Cb and Cr composed of eight bits each (hereinafter these data are together referred to as YCbCr data). This is because the planes need to be combined in the common data format. In this example, the YCbCr data format for moving picture data is commonly used. The YCbCr data that have been output from the RGB/YCbCr conversion circuit 30 and the opacity data al are input to a multiplication device 23. A resolution conversion circuit may be disposed between the ,RGB/YCbCr conversion circuit 30 and the multiplication device 23 so as to convert the resolution of the YCbCr data into another resolution. The multiplication device 23 multiplies the input YCbCr data by the opacity data ctl. The multiplied result is input to one input terminal of an addition device 24. The multiplication device 23 multiplies each of the luminance signal Y and the color difference signals Cb and Cr of the YCbCr data by the opacity data ctl. In addition, the complement (1 - al) of the opacity data al is supplied to the multiplication device 21. The multiplication device 21 multiplies the moving picture data that has been input from the 422/444 conversion circuit 20 by the complement (1 - al) of the opacity data al. The multiplied result is input to the other input terminal of the addition device 24. The addition device 24 adds the multiplied results of the multiplication devices 21 and 23. As a result, the video plane 10 and the presentation graphics plane 11 are combined. The added result of the addition device 24 is input to a multiplication device 25. Like the presentation graphics plane 11, the palette 26 outputs video data of the interactive graphics plane 12 as data of RGB (4 : 4 : 4). The video data are input to an RGB/YCbCr conversion circuit 27. When the color system of the video data that the interactive graphics plane 12 uses is RGB (4 : 4 : 4), the RGB/YCbCr conversion circuit 27 converts the color system of video data into YCbCr (4 : 4 : 4) and outputs YCbCr data. The YCbCr data that have been output from 5 RGB/YCbCr conversion circuit 27 are input to a multiplication device 28. A resolution conversion circuit may be disposed between the RGB/YCbCr conversion circuit 27 and the multiplication device 28 so as to convert the resolution of the YCbCr data into 10 another resolution. When opacity of alpha-blending has been designated in the palette 26 according to index values, designated opacity