_createRenderCmd() {
    if (cc._renderType === cc._RENDER_TYPE_CANVAS) return this._super();

    return new ShadowCastingLayer.WebGLRenderCmd(this);
  }
*************************************************************************
 ShadowCastingLayer
 - this layer will draw ALL shadows in the scene at once (for performance reasons)
 - there should only be ONE shadow casting layer in a scene
**************************************************************************