FIX tunnel building in sliced mode with click&drag

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6502 8aca7d54-2c30-db11-9de9-000461428c89
FIX: intialize factory pos and use pos_origin instead

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6503 8aca7d54-2c30-db11-9de9-000461428c89