code: function PageViewPort_clone(args) {
        args = args || {};
        var scale = 'scale' in args ? args.scale : this.scale;
        var rotation = 'rotation' in args ? args.rotation : this.rotation;
        return new PageViewport(this.viewBox.slice(), scale, rotation,
                                this.offsetX, this.offsetY, args.dontFlip);
      }
docs: Clones viewport with additional properties.
  @param args {Object} (optional) If specified, may contain the 'scale' or
  'rotation' properties to override the corresponding properties in
  the cloned viewport.
  @returns {PDFJS.PageViewport} Cloned viewport.