Custom scene graph types

[jambuttenshaw]

This pull request makes two small changes to make it easier to work with custom scene types:

1. Added SceneTypeFactory::CreateGraph, a factory method that makes it easy to supply a scene graph of a derived type. This is used in Scene::LoadWithExecutor to avoid having to override the loader simply to use a derived scene graph type. For anyone not interested in deriving from SceneGraph, no code changes are required.
2. Added the path to the scene file as a function parameter to LoadCustomData. This is useful if the custom data in the scene file specifies paths to resources relative to the scene file itself. This will require code changes for clients of LoadCustomData but nevertheless I think it would be useful.