Some GLX drivers, such as newer versions of the Intel drivers,
depend on X11 events coming from Xlib. So VLC cannot steal them from
the underlying XCB connection.

I have no time and will to duplicate the XCB code found in the GLX
plugin for windowing and events handling using Xlib. So lets make a
separate X connection with XCB. This is inefficient, but it works.

On the plus side, this patch removes the dependency on libx11-xcb.
So it is now possible to compile and use GLX with an Xtrans-based
Xlib (though libxcb must be present).