Old (scaled) images not reset if `big_image_size_threshold` increased or removed

Reported at: https://wordpress.org/support/topic/old-scaled-images-not-reset-if-big_image_size_threshold-increased-or-removed/

> If I use the big_image_size_threshold hook to increase or remove scaling of large images on a site with existing media, the plugin continues to uses the “scaled” images as the source, rather than looking at the “original_image” src to decide wether larger images should be created.

> In this scenario, if the big_image_size_threshold filter is used to disable the big image threshold, I believe the _wp_attached_file meta value should be changed to the “original_image” src URL prior to regenerating images.

> If the big_image_size_threshold filter is used to change the big image threshold, I believe the _wp_attached_file meta value should generate a new version of the ‘scaled’ image at the size defined by big_image_size_threshold.

No source files or tests are named. Start by tracing the plugin's image regeneration path and how it uses the big_image_size_threshold hook, original_image data, and _wp_attached_file metadata. Done means changing or removing the threshold causes regeneration to use the original image appropriately and creates the expected scaled version when needed.

Written by the indexing model from the issue text.