Add support for an extended style of _NET_WM_SYNC_REQUEST_COUNTER

If an application provides two values in _NET_WM_SYNC_REQUEST_COUNTER,
use that as a signal that the applications wants an extended behavior
where it can update the counter as well as the window manager. If the
application updates the counter to an odd value, updates of the
window are frozen until the counter is updated again to an even value.

https://bugzilla.gnome.org/show_bug.cgi?id=685463

src/core/display-private.h

@@ -222,6 +222,9 @@ struct _MetaDisplay
 #ifdef HAVE_XKB
   int         xkb_base_event_type;
   guint32     last_bell_time;
+#endif
+#ifdef HAVE_XSYNC
+  gint64      grab_sync_counter_wait_serial;
 #endif
   int	      grab_resize_timeout_id;