Big image size threshold should take into account registered image sizes.

[johnbillion]

The "big image" upper size threshold is set to 2560. If an image size is registered that has a width or height larger than this, then the image will be unexpectedly cropped to 2560.

The value that gets passed through to the big_image_size_threshold filter should be set to the maximum value of either 2560 or the largest width or height from all registered image sizes.

[psrpinto]

I submitted a patch which implements the solution suggested in the ticket description:

The value that gets passed through to the big_image_size_threshold filter should be set to the maximum value of either 2560 or the largest width or height from all registered image sizes.

However, I noticed that many of the default themes (e.g. twentytwentyone, twentytwenty, twentynineteen) set the post_thumbnail_size's height to 9999, which results in the threshold now being 9999.

This means the majority of pictures will not be cropped, and I'm not sure that was the intention behind the proposed solution.

Do you think such a high threshold makes sense @johnbillion?