konvajs async render 


   * batch draw. this function will not do immediate draw
   * but it will schedule drawing to next tick (requestAnimFrame)
   * @method
   * @name Konva.Layer#batchDraw
   * @return {Konva.Layer} this
 batchDraw() {
    if (!this._waitingForDraw) {
      this._waitingForDraw = true;
      Util.requestAnimFrame(() => {
        this.draw();
        this._waitingForDraw = false;