code: function(pixel) {
          $.console.assert(this.viewer,
              "[Viewport.windowToImageCoordinates] the viewport must have a viewer.");
          var viewerCoordinates = pixel.minus(
                  $.getElementPosition(this.viewer.element));
          return this.viewerElementToImageCoordinates(viewerCoordinates);
      }
docs: Convert pixel coordinates relative to the window to image coordinates.
  Note: not accurate with multi-image.
  @param {OpenSeadragon.Point} pixel
  @returns {OpenSeadragon.Point}