_render(scene, markTypes) {
      // perform spot updates and re-render markup
      if (this._dirtyCheck()) {
        if (this._dirtyAll) this._clearDefs();
        this.mark(this._root, scene, undefined, markTypes);
        domClear(this._root, 1);
      }
      this.defs();
      this._dirty = [];
      ++this._dirtyID;
      return this;
    }
Internal rendering method.
@param {object} scene - The root mark of a scenegraph to render.
@param {Array} markTypes - Array of the mark types to render.
                           If undefined, render all mark types