Limit custom image sizes to specific post types

[aaronholbrook]

Oftentimes I'll need a custom image size for featured images only for a specific post type and not for use in areas such as the blog or page post types.

This causes an undue strain on space usage for sites with frequent posting.

Ex. A blog site that puts out blog posts daily, but also has a directory section (CPT) with custom sizes for their featured image. Currently that means that all new blog posts now have additional image sizes created for them, which results in a large amount of wasted processing and space.

[helen]

Duplicate of #13372.