[RFC] Make the OpenGL renderer work on GL 2.0+ and GLES 2.0+

In order to do that, I introduced an OpenGL section in both config files, allowing the user to select the OpenGL version they want to use.
And to get the function corresponding to the context we got, move from glLoadGen to libepoxy.
This libepoxy change isn’t ready for merge yet, I didn’t change the CMakeLists.txt file to support MSVC or OSX, only pkg-config is currently supported.
I confirmed that there is GLES support on Qt, but it seems that it is only on very recent versions, and not supported with `QGLFormat` and `QGLWidget`. We might have to wait it out until we drop Qt 4 support.

Anyway, you should add a warning here saying that we ignored the "gles" option.