Now that MetaShapedTexture is not a ClutterActor anymore, it does
not make sense to make it a MetaCullable semi-implementation. This
is, naturally, a responsibility of MetaSurfaceActor, since now
MetaShapedTexture is a ClutterContent and as such, it only cares
about what to draw.
Move the MetaCullable implementation of MetaShapedTexture to
MetaSurfaceActor.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/409