Media: Update fallback previews to support `_wp_make_subsizes()`

Allows fallback previews (PDF by default, in core) to save image meta
after each size is generated to allow for resuming, and make sizes
immediately available.

Props azaozz, antpb, progremzion, mikeschroder.
See #48111.
Built from https://develop.svn.wordpress.org/trunk@46459


git-svn-id: https://core.svn.wordpress.org/trunk@46257 1a063a9b-81f0-0310-95a4-ce76da25c4cd