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?
Suggested alternative and simplified comment: "Fills active OpenGL texture with the given RGB color" (If you don't take the suggestion, please try to clarify this comment, because half of it doesn't really make sense to me, unfortunately)