Always show Background Image section heading and the opening table tag on...

Always show Background Image section heading and the opening table tag on Custom Background screen, to make in consistent with Custom Header and avoid broken markup.

props obenland.
fixes #26731.
Built from https://develop.svn.wordpress.org/trunk@26878


git-svn-id: https://core.svn.wordpress.org/trunk@26761 1a063a9b-81f0-0310-95a4-ce76da25c4cd