Introduce `has_image_size( $name )`, a utility function that helps avoid loading the `$_wp_additional_image_sizes` global.

Props mordauk.
Fixes #26951.


Built from https://develop.svn.wordpress.org/trunk@27128


git-svn-id: http://core.svn.wordpress.org/trunk@26995 1a063a9b-81f0-0310-95a4-ce76da25c4cd