Responsive images: fix calculations when determining whether to include...

Responsive images: fix calculations when determining whether to include particular image file in `srcset`.

Props joemcgill.
Fixes #34955 for trunk.
Built from https://develop.svn.wordpress.org/trunk@36031


git-svn-id: https://core.svn.wordpress.org/trunk@35996 1a063a9b-81f0-0310-95a4-ce76da25c4cd