Major rewrite to the animation interaction between the Engine and Scene.
There were a couple of minor bugs with the animation sequencing that I found I couldn't fix without breaking something else in the old system.

The Engine now queues up a number of animation "stages" in the Scene and only triggers the animation to start when necessary.
This involves a bit more code in the Scene and a bit less in the Engine, but the result is a whole lot less fragile and a whole lot more readable.

This change has most likely introduced some regressions.
More cleanup and changes to follow.