Opengl es 3.0 support

This contains an interface for es 3.0 functions and constants. I've implemented this interface in the lwjgl backend only.

I've also added some tests showing the more readily visible features of es 3.0, under gdx-tests/src/com/badlogic/gdx/tests/gles3

Everything is commented, but for the tests, some background in the techniques demonstrated is recommended.