Allow image styles to be saved to a different scheme to the original image

[yorkshire-pudding]

Description of the need

For photographers and artists, the original image may be part of their intellectual property that should only be available to users of a certain role (e.g. "Subscriber"). They would therefore use the private folder for these original images so they can be protected. However, a thumbnail and/or a watermarked larger image may be available to the public and so could be in the public files where checks aren't needed before delivering them to the browser.

Proposed solution

In the same way that image field can specify a scheme, the image style should allow the scheme to be changed. Ideally, this would be in the UI with a simple set of radio buttons.

Alternatives that have been considered

No modules that do this, partly because there are no hooks to change the scheme.

Drupal have done (and are doing) work in core to allow modules to alter the scheme and path of image styles; see the following issues to see the trail I followed:

• https://www.drupal.org/project/imagecache_actions/issues/1863720
• https://www.drupal.org/node/1903190
• https://www.drupal.org/project/drupal/issues/2027423
• https://www.drupal.org/project/drupal/issues/1358896

Additional information

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability to save image styles to public even if the original images are in private and vice versa.