(ArthurDenture/turfit) ADD: sdl2 backend

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6688 8aca7d54-2c30-db11-9de9-000461428c89
FIX: posbuttons must not translate

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6689 8aca7d54-2c30-db11-9de9-000461428c89
FIX: gaps in multithreaded view when screen width and number of threads aren't integral multiples

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6690 8aca7d54-2c30-db11-9de9-000461428c89
FIX: loading old games never reconstructed intermediate stops but got halthandle overflow

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6691 8aca7d54-2c30-db11-9de9-000461428c89
Only use player colors, if the image actually has some ... 

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6692 8aca7d54-2c30-db11-9de9-000461428c89
(AthurDenture) records in its own class

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6693 8aca7d54-2c30-db11-9de9-000461428c89
(AthurDenture) sounds in its own routine

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6694 8aca7d54-2c30-db11-9de9-000461428c89