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?
Please don't pass `texture` as modifyable here. I've had a long debate with @yuriks about this, and the tl;dr about my criticism is that it's an oxymoron to pass class members by non-const reference to a static method. Regardless of how valid that complaint is, it should not be necessary here, as explained in my other comment.