Fix OpenGL errors.

Fixes MovingBlocks/Terasology#461.

The switch statement requires GLSL 1.3, and OS X doesn't support it, so
fake it with if-then-else statements.

The graphics card in my Macbook Pro doesn't support FBOType.NO_COLOR
either, and I suspect this applies to multiple Mac models.

Tested on a MacBookPro5,3, with OS X 10.8.2.