actor: Move the ShaderData out of the private data

actor: Move the ShaderData out of the private data

Let's try and start reducing the size of ClutterActorPrivate by moving
some optional, out-of-band data from it to GObject data.

The ShaderData structure is a prime candidate for this migration: it
does not need to be inspected by the actor, and its relationship with an
actor is transient and optional.

By attaching it to the actor's instance through g_object_set_data() we
neatly tie its lifetime to the instance, and we don't have to care
cleaning it up in the finalize()/dispose() implementation of
ClutterActor itself.