FIX graphical glitches with high mountains

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6785 8aca7d54-2c30-db11-9de9-000461428c89
shuffle CODE around to ensure that get_ground_on_screen_coordinate does what it should do

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6786 8aca7d54-2c30-db11-9de9-000461428c89
CODE: get rid of karte_t::mi,mj finally

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6787 8aca7d54-2c30-db11-9de9-000461428c89