Twenty Seventeen: Additional default header image optimizations.

This is a follow-up to [39248] that applies a bit more compression to
the default header image in Twenty Seventeen.

Props Presskopp, lukecavanagh.
Fixes #38793.
Built from https://develop.svn.wordpress.org/trunk@39279


git-svn-id: https://core.svn.wordpress.org/trunk@39219 1a063a9b-81f0-0310-95a4-ce76da25c4cd