FIX: crash with minimap when deleting stops

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6337 8aca7d54-2c30-db11-9de9-000461428c89
CODE: optimizations of vehicle movement code: save calls to karte_t::lookup

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6338 8aca7d54-2c30-db11-9de9-000461428c89
CODE cleanup in movingobj

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6339 8aca7d54-2c30-db11-9de9-000461428c89
groundobj on every tile possible again

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6340 8aca7d54-2c30-db11-9de9-000461428c89