Since GLX and EGL are abstracted by Cogl the two backends are both
implementing everything using the Cogl API and they are almost
identical.

This updates the egl backend to support everything that the glx backend
supports. Now that EGL and GLX are abstracted by Cogl, the plan is that
we will squash the clutter-egl/glx backends into one. Since the EGL
backend in clutter can conditionally not depend on X11 we will use the
EGL backend as the starting point of our common backend.

https://bugzilla.gnome.org/show_bug.cgi?id=649826