FIX: correctly mark image dirty - take offsets into account

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7045 8aca7d54-2c30-db11-9de9-000461428c89
FIX: smoke offset after rotation, code cleanup

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7046 8aca7d54-2c30-db11-9de9-000461428c89