Evas GL common: Create OpenGL ES 3.x contexts by default

Evas GL common: Create OpenGL ES 3.x contexts by default

Automatically fallback to OpenGL ES 2.0 if OpenGL ES 3 is not supported.
This is a first step in trying to support GLES 3 for Evas GL.

This commit is also a wild test to see whether using GLES 3 contexts
by default will break anything. The theory says that GLES 3 is
backwards compatible with GLESv2.

So, if anything GL breaks for you... scream loudly!
But before reporting any bugs, please set the env variable:
- export EVAS_GL_DISABLE_GLES3=1

This does not add any requirement for GLESv3 support.