FIX: kartenboden_setzen makes gr invalid

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6892 8aca7d54-2c30-db11-9de9-000461428c89
FIX bound check to access water_hgt

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6893 8aca7d54-2c30-db11-9de9-000461428c89
FIX: sometimes north neighbour tile were not lowered correctly

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6894 8aca7d54-2c30-db11-9de9-000461428c89