code: function (zlevel, virtual) {
          if (this._singleCanvas && !this._needsManuallyCompositing) {
              zlevel = CANVAS_ZLEVEL;
          }
          var layer = this._layers[zlevel];
          if (!layer) {
              // Create a new layer
              layer = new Layer('zr_' + zlevel, this, this.dpr);
              layer.zlevel = zlevel;
              layer.__builtin__ = true;
  
              if (this._layerConfig[zlevel]) {
                  util.merge(layer, this._layerConfig[zlevel], true);
              }
  
              if (virtual) {
                  layer.virtual = virtual;
              }
  
              this.insertLayer(zlevel, layer);
  
              // Context is created after dom inserted to document
              // Or excanvas will get 0px clientWidth and clientHeight
              layer.initContext();
          }
  
          return layer;
      }
docs: 获取 zlevel 所在层，如果不存在则会创建一个新的层
  @param {number} zlevel
  @param {boolean} virtual Virtual layer will not be inserted into dom.
  @return {module:zrender/Layer}