compositor: Initiate cogl context field on creation

There will only ever be one context; just get it from clutter instead
of waiting for we interact with a framebuffer.

https://bugzilla.gnome.org/show_bug.cgi?id=768976

src/backends/meta-backend.c

@@ -671,7 +671,7 @@ static GSourceFuncs event_funcs = {
   event_dispatch
 };
 
-static ClutterBackend *
+ClutterBackend *
 meta_backend_get_clutter_backend (MetaBackend *backend)
 {
   MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);