Bug when zooming timeline too fast

Steps to reproduce:
1. Load https://cesiumjs.org/Cesium/Apps/HelloWorld.html
2. Put cursor over timeline and give it one fast scroll (two fingers on touchpad)
3. Try zooming a few more times and it should not zoom anymore

I think the bug is in the code below. It goes away if you make `timelineWheelDelta` constnat, but this decouples zooming speed from scrolling speed.

```
function createMouseWheelCallback(timeline) {
return function(e) {
var dy = e.wheelDeltaY || e.wheelDelta || (-e.detail);
timelineWheelDelta = Math.max(Math.min(Math.abs(dy), timelineWheelDelta), 1);
dy /= timelineWheelDelta;
timeline.zoomFrom(Math.pow(1.05, -dy));
};
}
```

Reproduce the issue in the linked Cesium HelloWorld application by scrolling rapidly over the timeline. Then locate createMouseWheelCallback and inspect how timelineWheelDelta changes between wheel events. Done means repeated fast zooming continues to work while scrolling speed still affects zoom speed; the payload names no test to run.

Written by the indexing model from the issue text.