x11: vdpau GLX interop needs X11 threads

Xlib is not thread-safe. Or actually it is, but it's an incomprehensible
hack that was added later, and which needs to be acitvated manually
(this makes no sense). And it appears that the vdpau accesses X from the
decoder thread if GLX interop is used (and not in any other situations -
this doesn't make too much sense either).

So, just call the magic function that enables Xlib thread-safety.