More consistent naming of slope differences for hang

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6968 8aca7d54-2c30-db11-9de9-000461428c89
Make grund_besch_t a little more robust against image registration

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6969 8aca7d54-2c30-db11-9de9-000461428c89