Consider adding stateful interface for glTF animation

Some users find the stateless Animator interface to be too low-level because it merely pushes a user-provided "elapsed time" into the glTF animation machinery. It is currently agnostic of concepts like play / pause / loop / reverse.

If we make this enhancement, users may need callback notifications as well.

We can either enhance Animator, or add a layer on top. I'm leaning towards the latter since callbacks often need to be expressed in a platform-specific (or language-specific) way.