Add implementation for color filling.

TODO: Get LCD Color Fill data from GPU

This is part of a bigger effort (http://3dbrew.org/wiki/GSPGPU:SetLcdForceBlack, https://gist.github.com/c1756269908b54782aef), but I can't figure out how I should go about reading and writing the values...everything I've tried leads to `Unknown Read32`/`Unknown Write32`. Any thoughts?
Perhaps a better assert would be `dbg_assert(pixel_stride % 3 == 0)`?