[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.
Put `glsl_header` as first parameter instead? Makes more sense to me.