[Feature Request] progressCallback should take img fetching into account

[MrSimmmons]

I just tried to implement a progress bar using the progressCallback method, and discovered it only gets called on page rendering. This feature could be better IMO because the render step is very quick taking < 1 second most of the time. However if you have a lot of images in the PDF, they can take quite a while to be fetched if being supplied as a URL.

The progressCallback never gets called during the img fetching which is often the longest part of the export and would create poor UX if a progress bar were to be implemented as it would sit on 0% for the majority of the export process.

It would be good if progressCallback were to be called during the entire export process instead of just the render step.