adjustZoom(delta, anchor) {
    this.adjustResolution(Math.pow(this.zoomFactor_, -delta), anchor);
  }
Adds a value to the view zoom level, optionally using an anchor. Any resolution
constraint will apply.
@param {number} delta Relative value to add to the zoom level.
@param {import("./coordinate.js").Coordinate} [anchor] The origin of the transformation.
@api