Flush outstanding stuff when clearing directly on a window

Clearing will directly modify the contents of the window, so we need
to flush any outstanding moves or double-buffering.
(cherry picked from commit 8c1ea38d84)

gdk/gdkwindow.c

@@ -4431,6 +4431,10 @@ gdk_window_clear_region_internal (GdkWindow *window,
 	  gdk_region_intersect (copy,
 				private->clip_region_with_children);
 
+
+	  /* Drawing directly to the window, flush anything outstanding to
+	     guarantee ordering. */
+	  gdk_window_flush (window);
 	  impl_iface->clear_region (window, copy, send_expose);
 
 	  gdk_region_destroy (copy);