Avoid an X error

We are getting the mime data destroy notify called when we
destroy the surface in finalize. Trying to set the XSync counters
at this time is a) pointless and b) yielding an X error because
the counters have already been destroyed.
To avoid this, unhook the damage tracking before destroying
the surface.

https://bugzilla.gnome.org/show_bug.cgi?id=760188

gdk/x11/gdkwindow-x11.c

@@ -1331,6 +1331,8 @@ gdk_x11_window_destroy (GdkWindow *window,
   if (toplevel)
     gdk_toplevel_x11_free_contents (GDK_WINDOW_DISPLAY (window), toplevel);
 
+  unhook_surface_changed (window);
+
   if (impl->cairo_surface)
     {
       cairo_surface_finish (impl->cairo_surface);