slight optimization: reduce use of get_2d

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6850 8aca7d54-2c30-db11-9de9-000461428c89
CODE rearrangement in vehikel_basis_t::fahre_basis to reduce overhead

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6851 8aca7d54-2c30-db11-9de9-000461428c89