Add temporary cogl-clutter.h to aid splitting out Cogl

This gives us a way to clearly track the internal Cogl API that Clutter
depends on. The aim is to split Cogl out from Clutter into a standalone
3D graphics API and eventually we want to get rid of any private
interfaces for Clutter so its useful to have a handle on that task.
Actually it's not as bad as I was expecting though.

clutter/clutter-backend.c

@@ -465,7 +465,7 @@ _clutter_backend_ensure_context (ClutterBackend *backend,
 
           clutter_actor_get_size (CLUTTER_ACTOR (stage), &width, &height);
 
-          _cogl_onscreen_clutter_backend_set_size (width, height);
+          cogl_onscreen_clutter_backend_set_size (width, height);
 
           /* Eventually we will have a separate CoglFramebuffer for
            * each stage and each one will track private projection