Add support for OpenGL 3 contexts
This required some further changes:
* GLX backend: Choose FB configs instead of X visuals
* GL2 renderer: Set up a VAO if necessary
- [DH] .gitignore
- [DH] Makefile
- [DH] eglbackend.cpp
- [DH] glutil_gl1.cpp
- [DH] glutil_gl2.cpp
- [DH] glxbackend.cpp
- [DH] glxbackend.h