tags.php with mode letter.
We can't rely on url_name because it always start with "_" when the tag looks
like "[lang...". Instead we compute a str2url on the name, but once it has been
rendered by Extended Description. To avoid useless computations, I have added a
cache on the tag_alpha_compare function. Tested on a 175 tags set, speed is
good.
git-svn-id: http://piwigo.org/svn/branches/2.3@12760 68402e56-0260-453c-a942-63ccdbb3a9ee