FIX: mark-image dirty offsets, fixes pak64 fountain glitch; more code cleanup

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7047 8aca7d54-2c30-db11-9de9-000461428c89
small change to documentation

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7048 8aca7d54-2c30-db11-9de9-000461428c89