function getViewportSize(options, pswp) {
  if (options.getViewportSizeFn) {
    const newViewportSize = options.getViewportSizeFn(options, pswp);

    if (newViewportSize) {
      return newViewportSize;
    }
  }

  return {
    x: document.documentElement.clientWidth,
    // TODO: height on mobile is very incosistent due to toolbar
    // find a way to improve this
    //
    // document.documentElement.clientHeight - doesn't seem to work well
    y: window.innerHeight
  };
}
@param {PhotoSwipeOptions} options
@param {PhotoSwipeBase} pswp
@returns {Point}