Add an option to never zoom larger than the original image size

[JPustkuchen]

First, let me say "thank you" very much for this wonderful library!!

I think there might be a similar issue or request in the list of issues, I tried to find a matching one, but wasn't really successful. We're experiencing the following:

Context:
Clicking on a photoswipe image that use one of the available string view modes:

• fit
• fill

Problem:
When using photoswipe on an image that is for example 800x800px and using the "fit" option, zooming works perfectly on smaller screens, e.g. Smartphones. The image fits the viewport and is not shown larger than its original size.
Having a larger, e.g. desktop screen, the image is also zoomed to the boundaries, but for that reason zoomed larger than the image is and therefore becomes blurry!

Setting the zoom level to a hard value like "1" also isn't a solution because then the image is zoomed in heavily depending on the screen size.

The logic would be: Fit the screen, but do not display the image larger than it is!

Proposed solution:
While though custom code and a callback function one could solve this on your own, it would be very handy to have a predefined zoom level string to ensure this is not zoomed larger than the original image initially.

Additionally or as alternative, a boolean option to prevent overscale could be added, so that users can not zoom larger than the original image size.

What do you think?

Of course, we'd be willing to help to implement this, if you like this, and we can make a plan together!

Reproduction:
You can see this in the demos.