Upload: When an image was scaled because it is larger than the big image...

Upload: When an image was scaled because it is larger than the big image threshold, use the originally uploaded image's dimensions in `wp_get_missing_image_subsizes()`. Fixes an edge case/inconsistent behaviour when a registered image sub-size is also larger than the big image threshold.

Props desrosj, azaozz.
Fixes #48518 for trunk.
Built from https://develop.svn.wordpress.org/trunk@46677


git-svn-id: https://core.svn.wordpress.org/trunk@46477 1a063a9b-81f0-0310-95a4-ce76da25c4cd