Fix jitter in React editor

Fixes #2797
Refs #2769

I previously thought that using `setImmediate` instead of `nextTick` fixed our jitter issues when requesting updates from animation frames, but it seems I was wrong. This PR returns to a strategy of updating synchronously in animation frames, so we never mix calls to `requestAnimationFrame` with `setImmediate`. It seems to fix the jitter problems.

@Flannelhead: Could you try this out when it lands to see if it addresses #2769 on Linux?