feature 2541 multisize

- nicer presentation on picture.php
- added a maintenance purge derivatives action

git-svn-id: http://piwigo.org/svn/trunk@12797 68402e56-0260-453c-a942-63ccdbb3a9ee

include/derivative_params.inc.php

@@ -19,6 +19,11 @@
 // | USA.                                                                  |
 // +-----------------------------------------------------------------------+
 
+function derivative_to_url($t)
+{
+  return substr($t, 0, 2);
+}
+
 function size_to_url($s)
 {
   if ($s[0]==$s[1])