This is needed to protect the Xlib internal state. Xlib may still be
used by user interface thread, the GLX video output thread(s) and
PulseAudio -audio thread- while it initializes.

Fortunately, we do not need to lock X11 displays as every thread uses
its own. Hopefully Xlib is not buggy in that respect, but I am not so
sure :(

This should fix LP #419915.