some less indentation in grund_t to make it a little compacter

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6504 8aca7d54-2c30-db11-9de9-000461428c89
Draw earth border using the existing slope images (not double height tetsted yet) and optional the outside tile

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6505 8aca7d54-2c30-db11-9de9-000461428c89