FIX: gui tile dirty marking respect clipping rectangle

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6518 8aca7d54-2c30-db11-9de9-000461428c89
Merge branch 'ncn-devel' of git://github.com/neroden/simutrans.git into dingliste-cleanup