FIX: delta_t could be negative when ticks were overflowing

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6551 8aca7d54-2c30-db11-9de9-000461428c89
FIX: do not try to delete pedestrian from sync-list when it was not added before

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6552 8aca7d54-2c30-db11-9de9-000461428c89
FIX creation of alpha-tiles with non-matching image sizes (eg pak192.comic)

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6553 8aca7d54-2c30-db11-9de9-000461428c89