Optimize getAnimations() calls

[nickschot]

Two points we need to consider:

1. Using document.getAnimations() to pause all animations in willTransition may be more efficient than doing it per sprite as we'd issue a single call to the DOM
2. getAnimations() returns all animations, including CSS transitions/animations. Not sure yet if we can distinguish between them and if it matters.

[nickschot]

(1) has been implemented in #31 in a very basic way

[nickschot]

This may not be needed if we time our DOM reads/writes properly.