FIX: initialize offsets with dummy values for COLOUR_DEPTH==0

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6562 8aca7d54-2c30-db11-9de9-000461428c89
FIX crash in image_reader with COLOUR_DEPTH == 0

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6563 8aca7d54-2c30-db11-9de9-000461428c89
FIX: use delete_node to delete stale besch's

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6564 8aca7d54-2c30-db11-9de9-000461428c89