Index footprint images, convert Spinda spots to .png files

[GriffinRichards]

• The pokémon footprint images were grayscale, which meant that A: they were colored incorrectly, and B: if someone edited then saved one as an indexed image it would likely rearrange the palette, and cause the footprint to look incorrect in-game. They are now correctly indexed/colored.
• Spinda's spots are a bit matrix in a different 1bpp format than the footprints (a plain matrix, rather than an 8x8 tiled one). gbagfx was updated to support this other format so that Spinda's spots can be edited directly as .png files.

For those updating their own grayscale footprints, the .png files were regenerated using gbagfx:

tools/gbagfx/gbagfx graphics/pokemon/*/footprint.1bpp graphics/pokemon/*/footprint.png -palette temp.pal -width 2

where * is each species folder name and temp.pal is the following palette:

JASC-PAL
0100
2
255 255 255
98 98 98