gl/x11: use xcb instead of libX11

- xcb is supposedly thread-safe!

videotestsrc ! glimagesink now doesn't spuriously result in a
'call XInitThreads()' error however if anybody else is using X11,
then XInitThreads() still needs to be called and multiple glimagesink's
still need XInitThreads().

Everything still takes libX11 handles as they are compatible with the xcb
variants.  Unfortunately we cannot move fully over to xcb due to GLX being
entirely based on Xlib.  It's also impossible to transform a xcb_connection
to a Display which means we require X11 handles.

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