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?
My impression is that the framebuffer texture will only be resized when changing between LCD filled and not filled. Between two frames where it is being filled, the texture size should still be 1x1, so this branch shouldn't be taken.