- Introduces `wp_get_missing_image_subsizes()` and `wp_update_image_subsizes()` to generate image sub-sizes that are missing or were not created after the upload.
- Adds a way to display errors that happened while creating sub-sizes.
- Introduces `wp_create_image_subsizes()` intended for use after an image was uploaded. It saves/updates the image metadata immediately after each sub-size is created. This fixes the (long standing) problem when some of the sub-size image files were created but there was a timeout or an error and the metadata was not saved. Until now such uploads were considered "failed" which usually resulted in the user trying to upload the same image again, creating even more "orphan" image files.
Note that the patch also includes some unrelated WPCS fixes.
Props mikeschroder, azaozz.
See #40439.
git-svn-id: https://develop.svn.wordpress.org/trunk@45538 602fd350-edb4-49c9-b593-d223f7449a82
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