code: function (canvasEl) {
        this.lowerCanvasEl = fabric.util.getById(canvasEl) || this._createCanvasElement();
        this._initCanvasElement(this.lowerCanvasEl);
  
        fabric.util.addClass(this.lowerCanvasEl, 'lower-canvas');
  
        if (this.interactive) {
          this._applyCanvasStyle(this.lowerCanvasEl);
        }
  
        this.contextContainer = this.lowerCanvasEl.getContext('2d');
      }
docs: Creates a secondary canvas
  @method _createLowerCanvas