message: remove additived animator if it's finished for memory effeciency
diff: @@ -787,6 +787,11 @@ export default class Animator<T> {
  loop: this._loop,
  delay: this._delay,
  onframe(target: T, percent: number) {
  + // Remove additived animator if it's finished.
  + // For the purpose of memory effeciency.
  + if (self._additiveAnimator && !self._additiveAnimator._clip) {
  + self._additiveAnimator = null;
  + }
  for (let i = 0; i < tracks.length; i++) {
  tracks[i].step(target, percent);
  }
  @@ -812,7 +817,6 @@ export default class Animator<T> {
  }
  }
  else {
  -
  // This optimization will help the case that in the upper application
  // the view may be refreshed frequently, where animation will be
  // called repeatly but nothing changed.
  