x11: Add call to XInitThreads()

The Mesa Vulkan drivers need XInitThreads() being called, because their
implementation has to use threads.

And I don't want to make the call depend on if Vulkan is compiled in
because that makes GTK's X11 behavior depend on compile-time flags, so
it's always called.

gdk/x11/gdkdisplay-x11.c

@@ -1367,6 +1367,8 @@ _gdk_x11_display_open (const gchar *display_name)
   gint ignore;
   gint maj, min;
 
+  XInitThreads ();
+
   xdisplay = XOpenDisplay (display_name);
   if (!xdisplay)
     return NULL;