Make compositor private a GObject than a void pointer

Requiring the compositor private object to be a GObject allows the
return value of meta_window_get_compositor_private() to be used
via gobject-introspection, since the type and memory management
can be determined.

This allows a reverse mapping from MetaWindow to MutterWindow.

- Change meta_window_get/set_compositor_private to use GObject
  rather than void *
- Adapt code in compositor-mutter.c to match.

src/core/window-private.h

@@ -360,7 +360,7 @@ struct _MetaWindow
   MetaGroup *group;
 
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  void *compositor_private;
+  GObject *compositor_private;
 #endif
 };
 

src/core/window.c

@@ -8707,7 +8707,15 @@ meta_window_get_description (MetaWindow *window)
   return window->desc;
 }
 
-void *
+/**
+ * meta_window_get_compositor_private:
+ * @window: a #MetaWindow
+ *
+ * Gets the compositor's wrapper object for @window.
+ *
+ * Return value: (transfer none): the wrapper object.
+ **/
+GObject *
 meta_window_get_compositor_private (MetaWindow *window)
 {
   if (!window)
@@ -8716,7 +8724,7 @@ meta_window_get_compositor_private (MetaWindow *window)
 }
 
 void
-meta_window_set_compositor_private (MetaWindow *window, void *priv)
+meta_window_set_compositor_private (MetaWindow *window, GObject *priv)
 {
   if (!window)
     return;