Media: Allow PDF fallbacks filter to process custom sizes.

This fixes an oversight in [39246], which added a hook for filtering
the array of sizes used for PDF thumbnails, but failed to provide a way
for sizes added through `add_image_size()` to be processed.

Props gitlost.
Fixes #39231. See #38594.
Built from https://develop.svn.wordpress.org/trunk@39617


git-svn-id: https://core.svn.wordpress.org/trunk@39557 1a063a9b-81f0-0310-95a4-ce76da25c4cd