Part 2 of the display code

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6644 8aca7d54-2c30-db11-9de9-000461428c89
FIX: actually use pthread barriers

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6645 8aca7d54-2c30-db11-9de9-000461428c89