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.

video/out/x11_common.c

@@ -399,6 +399,8 @@ int vo_x11_init(struct vo *vo)
 
     assert(!vo->x11);
 
+    XInitThreads();
+
     struct vo_x11_state *x11 = talloc_ptrtype(NULL, x11);
     *x11 = (struct vo_x11_state){
         .log = mp_log_new(x11, vo->log, "x11"),