clutter/actor: Unref the root node if set as child
When setting the root node as child of a clip or transform node, we add a new reference to it, without removing the one that we've previously added when getting it from the actor node (and that won't ever be unset by the auto-pointer since the root_node is re-associated).
So, once we add the root node as child and re-define it, unref it.
Fixes #908 (closed)
Edited by Marco Trevisan