Remove MetaCompositor virtualization

Now that we only have one compositor, there's no reason to access the
compositor functions through a vtable. Remove the MetaCompositor virtualization
and make the clutter code implement the meta_compositor_* functions
directly.

Move the checks for the compositor being NULL from the vtable wrappers
to the calling code (most of them were already there, so just a few
needed to be added)

Note: the compositor is actually hard-coded on at the moment and the plan
  is to remove the non-composited code entirely, so the checks are
  added only to keep things neat: they have no practical effect.

http://bugzilla.gnome.org/show_bug.cgi?id=581813

src/core/stack-tracker.c

@@ -686,9 +686,10 @@ meta_stack_tracker_sync_stack (MetaStackTracker *tracker)
         meta_windows = g_list_prepend (meta_windows, meta_window);
     }
 
-  meta_compositor_sync_stack (tracker->screen->display->compositor,
-			      tracker->screen,
-			      meta_windows);
+  if (tracker->screen->display->compositor)
+    meta_compositor_sync_stack (tracker->screen->display->compositor,
+                                tracker->screen,
+                                meta_windows);
   g_list_free (meta_windows);
 
   meta_screen_restacked (tracker->screen);