Upload:

- Run the `wp_generate_attachment_metadata` filter at the end in `wp_update_image_subsizes()` when new metadata was generated and additional image sub-sizes were created.
- Add another arg in the `wp_generate_attachment_metadata` filter for additional context.
- Fix inline docs and ensure the new image meta is always saved before starting post-processing.

Fixes #48472 for trunk.
Built from https://develop.svn.wordpress.org/trunk@46621


git-svn-id: https://core.svn.wordpress.org/trunk@46418 1a063a9b-81f0-0310-95a4-ce76da25c4cd