Add color range option to gbagfx for palette exporting

[huderlem]

graphics/pokenav/region_map.png is a 256-color image, but its palette is currently not auto-exported by the build. This is because the game is clever, and only loads colors 112-143 into BG pal slots 7 and 8. If gbagfx could be configured to export only a certain color range when converting .png -> gbapal`, this could be an automatic process.

[huderlem]

Another, probably better, option is to only INCBIN the desired 32-color range from the 256-color palette.

[GriffinRichards]

For posterity (so they can also be generated when this is resolved), similar deal with the 8bpp .png/.pal pairs here https://github.com/pret/pokefirered/tree/master/graphics/oak_speech