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.c

@@ -53,9 +53,7 @@
 #include <X11/extensions/shape.h>
 #endif
 
-#ifdef HAVE_COMPOSITE_EXTENSIONS
 #include <X11/extensions/Xcomposite.h>
-#endif
 
 static int destroying_windows_disallowed = 0;
 
@@ -463,14 +461,12 @@ meta_window_new_with_attrs (MetaDisplay       *display,
        /* any windows created via meta_create_offscreen_window: */
        (attrs->x == -100 && attrs->y == -100
 	&& attrs->width == 1 && attrs->height == 1) ||
-#ifdef HAVE_COMPOSITE_EXTENSIONS
        xwindow == screen->wm_cm_selection_window ||
        xwindow == screen->guard_window ||
        (display->compositor &&
         xwindow == XCompositeGetOverlayWindow (display->xdisplay,
 					       screen->xroot)
        )
-#endif
       )
      ) {
     meta_verbose ("Not managing our own windows\n");