Use the lowest shader precision required.

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

Vertex positions are highp, texture UV coords are mediump, colors are
lowp.
Remove unused method.
Internal tweaks, renaming and OCD optimization.
Don't show layers that currently have no symbol.