gdk_window_end_paint: Flush destination surface after paint

It seems in cairo 1.14 we need this after having painted an image surface
to a X11 window surface (i.e. with GDK_RENDERING=image).

gdk/gdkwindow.c

@@ -3117,6 +3117,8 @@ gdk_window_end_paint (GdkWindow *window)
           cairo_paint (cr);
 
           cairo_destroy (cr);
+
+          cairo_surface_flush (surface);
         }
     }