So far the OpenGL is unsafe check functionality in Compositor disabled
OpenGL compositing if glx is not available and we are in standalone X11
mode.

This is technically no longer correct for quite some time. Just because
GLX is not available doesn't mean that OpenGL doesn't work. We have an
EGL backend. So let's try to use that if glx is not available.

This change removes the check completely from Compositor. Instead the
standalone x11 plugin checks whether glx is available prior to createing
the glx backend. If not available it falls through to the egl backend.