There can be only one compositor engine

Mutter is a Clutter-based compositing manager. So, remove the code for
the XRender-based compositor, and make it mandatory to have XComposite,
XRender and Clutter.

Run-time support for non-composited operation is left for now.

* src/compositor/mutter/: Move files from this subdirectory into
  the main compositor/ directory.

* compositor/compositor-xrender.ccompositor/compositor-xrender.h:
  Remove

* include/compositor-clutter.h: Remove this stray file, it had been
  replaced with compositor-mutter.h some time back.

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

src/core/window-private.h

@@ -358,9 +358,7 @@ struct _MetaWindow
   /* maintained by group.c */
   MetaGroup *group;
 
-#ifdef HAVE_COMPOSITE_EXTENSIONS
   GObject *compositor_private;
-#endif
 };
 
 struct _MetaWindowClass