Editing images only regenerates built in image sizes and not custom image sizes.

[hexpen]

Steps to reproduce:

1. Add image sizes using add_image_size (both cropped and uncropped perform the same).

2. Add New under media and upload a picture. (At this point all image sizes are generated correctly)

3. Edit newly uploaded image and perform an edit action such as crop or rotate (Edit -> Edit Image)

4. Save edit changes and update the attachment.

Now only full size, large, medium, and thumbnail images are regenerated with the new edit and the custom image sizes are skipped.

[markoheijnen]

Duplicate of #19889.