x11: Redo choice between EGL and GLX

x11: Redo choice between EGL and GLX

We try EGL first, but are very picky about what we accept.
If that fails, we try to go with GLX instead.
And if that also fails, we try EGL again, but this time accept anything.

The idea here is that EGL is the preferred method going forward, but GLX is
the tried and tested method that we know works. So if we detect issues with
EGL, we want to avoid using it in favor of GLX.

Also add a GDK_DEBUG=gl-egl option to force EGL at all costs and not try
GLX.