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?
Doesn't really matter, I'd say, as it depends on how you look at it - `pixel_stride * 3 == framebuffer.stride` is what we actually care about for our algorithm to work, while `framebuffer.stride % 3 == 0` is the obvious precondition for the former condition to be true.