start

Starts the renderer and registers for frameCallbacks.

Goal: When we trigger start, doFrame gets called once per frame until we stop or the animation finishes.

Gotchas:

• scheduleFrame triggers callbacks to doFrame which in turn schedules more frames

• If we call scheduleFrame multiple times we enter multiple parallel animations loops

• To avoid this we check isPlaying and deregister FrameCallbacks when stop is called by users