gtk-clutter-util: Merge fixes from post_parse_hook

Get these two methods back in sync.

https://bugzilla.gnome.org/show_bug.cgi?id=695737
gtk-clutter-util: Put the shared initializion logic in a shared function

https://bugzilla.gnome.org/show_bug.cgi?id=695737
gdk-clutter-util: Add Wayland support to init

Pretty useless, but gets the code running a little bit further.

https://bugzilla.gnome.org/show_bug.cgi?id=695737
wayland: Use the subsurface protocol to implement GtkClutterEmbed

The subsurface protocol lets us "embed" one surface within another. The
compositor will compose the two surfaces together to create a single
window. We use it here to take the surface we get from Clutter and make
that into a subsurface and then associate that subsurface with the main
surface coming from GTK+.

https://bugzilla.gnome.org/show_bug.cgi?id=695737