Debug tiles in tilemaps with GPU render.

The code is extremely ugly and it needs some serious love. It's just a prof of concept for now, but it's not good. When showing debug tiles, the framerate dropped from 30 to 20 because the tilemap is being rendered twice.

I've left several TODOs and clues to refactor the code later.
Fix debug tile drawing in blitting render.
Tweak render initialization and improve some docs.
Rename render methods to mimic BitmapData.

Several method in the FlxRender class were renamed to better mimic BitmapData method names. I did that to make the new rendering pipeline look more similar to the old one Flixel was using.