function lazyLoadSlide(index, instance) {
  const itemData = instance.getItemData(index);

  if (instance.dispatch('lazyLoadSlide', {
    index,
    itemData
  }).defaultPrevented) {
    return;
  }

  return lazyLoadData(itemData, instance, index);
}
Lazy-loads specific slide.
This function is used both by Lightbox and PhotoSwipe core,
thus it can be called before dialog is opened.

By default, it loads image based on viewport size and initial zoom level.

@param {number} index Slide index
@param {PhotoSwipeBase} instance PhotoSwipe or PhotoSwipeLightbox eventable instance
@returns {Content | undefined}