message: add setSleepAfterStill to provide the abitility to turn off auto sleep.
diff: @@ -77,6 +77,8 @@ class ZRender {
  handler: Handler
  animation: Animation
  + private _sleepAfterStill = 10;
  +
  private _stillFrameAccum = 0;
  private _needsRefresh = true
  @@ -251,13 +253,22 @@ class ZRender {
  else {
  this._stillFrameAccum++;
  + const sleepAfterStill = this._sleepAfterStill;
  // Stop the animiation after still for 10 frames.
  - if (this._stillFrameAccum > 10) {
  + if (sleepAfterStill > 0 && this._stillFrameAccum > sleepAfterStill) {
  this.animation.stop();
  }
  }
  }
  + /**
  + * Set sleep after still for frames.
  + * Disable auto sleep when it's 0.
  + */
  + setSleepAfterStill(stillFramesCount: number) {
  + this._sleepAfterStill = stillFramesCount;
  + }
  +
  /**
  * Wake up animation loop. But not render.
  */
  