The problem here is that setting this to `false` and `true` repeatedly inside a single animation frame could kick off multiple render loops.  We're assuming that this will stay `false` long enough that the current loop will fire and fizzle one last time.

Instead, we could have a private boolean tracking whether the render loop is active or not, that would get set to false on the next animation frame after the user turns this off.  That way we could be certain that the previous loop has ended.  The downside is, if the render loop crashed, the value would never get set false, and you could never start a new loop.  But, that's the situation master is in right now.  Not sure if there's a better plan.  I think the official spec of `requestAnimationFrame` returns a handle to the request that can be cancelled directly, but of course we're using polyfills and vendor-prefixed versions of that function that may not follow the spec correctly there.