Do not allow to add duplicated renderer into actor

Do not allow to add duplicated renderer into actor

Internal::Actor allow to add same renderer, but SceneGraph::Node doesn't allow.

If we do something like this, the logic might be breakdown.

actor.AddRenderer(renderer);
actor.AddRenderer(renderer);
actor.RemoveRenderer(renderer);

///< actor will have renderer, but actor's node doesn't have renderer.

To match with node's logic, let we don't allow to push-back the duplicated renderer.
Instead, just check the duplication and return the index.

Change-Id: I689896171b3df5d140089c3b99f44e1d3f0f8722
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>