when we use viewer.destroy after viewer.flyTo(some entity or dataSource)，will cause memory leaks

[itfsw]

Sandcastle example: Example
Browser: Chrome

1. when use zoomTo will never case;
2. use flyTo will like this:
   
3. we guess something happened at scene.postRender -> updateZoomTarget -> viewer._zoomIsFlight -> camera.flyToBoundingSphere -> cancel when viewer.destroy
   so we try

viewer.flyTo(xxx).when(()=>{
    viewer._zoomIsFlight  = false
})

before use destroy, it still cannot free entity object，so it may happend when use flyTo method

[OmarShehata]

Thanks for the Sandcastle @itfsw . I can see by capturing a heap snapshot, with the zoomTo and without the zoomTo, after they complete, that in one case if you filter for "Entity" you'll see the memory still exists, and the other it does not.

I think this is worth looking more into.

[lLeiyi]

Cesium.TweenCollection.prototype.update = function (time) {
  const tweens = this._tweens;

  let i = 0;
  time = defined(time)
    ? time / TimeConstants.SECONDS_PER_MILLISECOND
    : getTimestamp();
  while (i < tweens.length) {
    const tween = tweens[i];
    const tweenjs = tween.tweenjs;

    if (tween.needsStart) {
      tween.needsStart = false;
      tweenjs.start(time);
    } else if (tweenjs.update(time)) {
      i++;
    } else {
      tweenjs.stop();
      tweenjs._group.remove(tweenjs); //important
      tweens.splice(i, 1);
    }
  }
};

I find in TweenCollection.update tweenjs.stop(); can not clear _group, then _group will continue to grow and there is no way to release. _group retains the reference chain of the entity. So I manually cleared _group.