Anchor texturize to shortcodes to improve regex efficiency.

props miqrogroove.
see #29557 for segfault issues.

Built from https://develop.svn.wordpress.org/trunk@30449


git-svn-id: http://core.svn.wordpress.org/trunk@30443 1a063a9b-81f0-0310-95a4-ce76da25c4cd