Do not leak a cairo region

This memory leak fix was present in GTK+ 3 since 3.10. We forgot
to backport it to GTK+ 2.

https://bugzilla.gnome.org/show_bug.cgi?id=738837

gdk/gdkwindow.c

@@ -8747,6 +8747,8 @@ do_child_shapes (GdkWindow *window,
     gdk_region_subtract (region, private->shape);
 
   gdk_window_shape_combine_region (window, region, 0, 0);
+
+  cairo_region_destroy (region);
 }
 
 /**