Add API to restrict the windowing backend to load

In situations when the default backend would fail (for example
when compiled with X11 support but run without DISPLAY), or
when the application is using backend specific code, it makes
sense to let the application choose the backend explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=707869

clutter/clutter-backend-private.h

@@ -99,6 +99,8 @@ struct _ClutterBackendClass
   void (* settings_changed)   (ClutterBackend *backend);
 };
 
+ClutterBackend *        _clutter_create_backend                         (void);
+
 ClutterStageWindow *    _clutter_backend_create_stage                   (ClutterBackend         *backend,
                                                                          ClutterStage           *wrapper,
                                                                          GError                **error);