wp_get_missing_image_subsizes returns thumbnail sizes for animated GIFs

[ianmjones]

While thumbnails are not created for animated GIFs to avoid the problem of users picking a thumbnail size to insert into their content and unexpectedly getting a static image instead of an animated GIF, they are generated for static GIFs.

The wp_get_missing_image_subsizes function however recognizes animated GIFs as an image format that can have subsizes generated, and so returns a list of missing thumbnail sizes that will never be created.

This causes problems for various plugins when checking whether all required thumbnails exist before commencing processing the GIF etc.