TinyMCE: fix problems with undo/redo after resizing an image several times. Fixes #28389.

git-svn-id: https://develop.svn.wordpress.org/trunk@28614 602fd350-edb4-49c9-b593-d223f7449a82
Regex should be non-greedy, see #28389

git-svn-id: https://develop.svn.wordpress.org/trunk@28615 602fd350-edb4-49c9-b593-d223f7449a82