Optimized background display, improves performance lost to my previous patch.

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6430 8aca7d54-2c30-db11-9de9-000461428c89
CODE: get rid of some unused height-scaling macros

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6431 8aca7d54-2c30-db11-9de9-000461428c89
remove unused variables

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6432 8aca7d54-2c30-db11-9de9-000461428c89
squelch signed-unsigned comparison warnings

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6433 8aca7d54-2c30-db11-9de9-000461428c89
Make karte_t::move_cursor compatible with DOUBLE_GROUNDS

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6434 8aca7d54-2c30-db11-9de9-000461428c89