Adds a compositor_private member to MetaWindow

Since we often need to find a MutterWindow corresponding to a MetaWindow
this make it a simple de-reference of a compositor private pointer.

src/core/window-private.h

@@ -348,6 +348,10 @@ struct _MetaWindow
 
   /* maintained by group.c */
   MetaGroup *group;
+
+#ifdef HAVE_COMPOSITE_EXTENSIONS
+  void *compositor_private;
+#endif
 };
 
 /* These differ from window->has_foo_func in that they consider