Enable conversion of image sizes that will not fit a tile

[HorstBaerbel]

Thanks for the great tool. I would love to use it more as it seems to be the only actively developed GBA image converter, but: When I have data for sprites/objects bigger than 64x64 I find it very convenient to leave the bitmap data for them in one piece. I simply convert the image to .c and let my routine sort it out when loading the data into individual tiles at runtime. This is not possible with nin10kit. All tile data that does not have a proper size gives an error.

nin10kit --mode=sprites --bpp=4 --transparent=0 ../../data/image_192x64_16 image_192x64_16.png I[00:27:57.661]Exporting to ../../data/image_192x64_16 symbol base name is image_192x64_16 I[00:27:57.661]Reading image image_192x64_16.png I[00:27:57.669]Using GBA exporter mode SPRITES F[00:27:57.669]Invalid sprite size for image image_192x64_16, (192 64) Please fix

I'd prefer a warning (or a "--force" flag), because I know what I'm doing. As a workaround I tried converting my data in bitmap mode using "-mode=4 -bpp=4", but nin10kit was smart and converted the color data to 256 colors, not 16 colors. So no cigar. If that would work, that would be fine with me...