higher privacy level than user privacy level.
For an acceptable solution at performance level, I have implemented a cache:
for a given user, each album has a representative_picture_id. This cache also
avoids to perform numerous "order by rand()" SQL queries which is the case
when $conf['allow_random_representative'] = true;
git-svn-id: http://piwigo.org/svn/trunk@8802 68402e56-0260-453c-a942-63ccdbb3a9ee
