window: Pass the grab timestamp when updating the resize grab

Since XChangeActivePointerGrab doesn't have a direct equivalent
in XI2, we need to make sure we don't re-grab with a newer
tiemstamp.

src/core/display-private.h

@@ -211,6 +211,7 @@ struct _MetaDisplay
   GList*      grab_old_window_stacking;
   MetaEdgeResistanceData *grab_edge_resistance_data;
   unsigned int grab_last_user_action_was_snap;
+  guint32     grab_timestamp;
 
   /* we use property updates as sentinels for certain window focus events
    * to avoid some race conditions on EnterNotify events