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.
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);
    }