Implement support for compressed textures in GL

[DarthMike]

See best practices in Apple docs. Of special importance for iOS.
Should support at least PVRTC

https://developer.apple.com/library/ios/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/TechniquesForWorkingWithTextureData/TechniquesForWorkingWithTextureData.html