Use minimal fragment shader precision.

As recommended by https://developer.apple.com/library/IOS/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/BestPracticesforShaders/BestPracticesforShaders.html#//apple_ref/doc/uid/TP40008793-CH7-SW7

This actually squeezes a few FPS out of iOS with no noticeable quality
loss. We can probably do this for vertex shaders too, but the most
benefit is going to be in the frag shader.