_getMax() {
    // max zoom level is x4 from "fit state",
    // used for zoom gesture and ctrl/trackpad zoom
    return this._parseZoomLevelOption('max') || Math.max(1, this.fit * 4);
  }
Maximum zoom level when user zooms
via zoom/pinch gesture,
via cmd/ctrl-wheel or via trackpad.

@private
@return {number}