Send _NET_WM_FRAME_DRAWN messages

When the application provides the extended second counter for
_NET_WM_SYNC_REQUEST, send a client message with completion
information after the next redraw after each counter update
by the application.

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

src/core/window-private.h

@@ -356,6 +356,10 @@ struct _MetaWindow
    * also handles application frames */
   guint extended_sync_request_counter : 1;
 
+  /* if TRUE, we still need to send a _NET_WM_FRAME_DRAWN message for the
+   * last update the sync request counter */
+  guint needs_frame_drawn : 1;
+
   /* Note: can be NULL */
   GSList *struts;