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/frame.c

@@ -29,9 +29,7 @@
 #include "errors.h"
 #include "keybindings-private.h"
 
-#ifdef HAVE_RENDER
 #include <X11/extensions/Xrender.h>
-#endif
 
 #define EVENT_MASK (SubstructureRedirectMask |                     \
                     StructureNotifyMask | SubstructureNotifyMask | \