(Kierongreen) double slope border support

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6509 8aca7d54-2c30-db11-9de9-000461428c89
FIX: draw the border also for tiles not on the screen

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6510 8aca7d54-2c30-db11-9de9-000461428c89