FIX: a diagonal river may not find a way on water

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6865 8aca7d54-2c30-db11-9de9-000461428c89
FIX: could not lower terrain below sea level when neighbouring tiles were occupied

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6866 8aca7d54-2c30-db11-9de9-000461428c89