FIX: schedule window (lines) fixed

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6991 8aca7d54-2c30-db11-9de9-000461428c89
FIX: do not use single-slope images if double-slope images are available but empty

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6992 8aca7d54-2c30-db11-9de9-000461428c89