CHG: refactored all the camera viewport related code out of simworld, to viewport_t.

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6816 8aca7d54-2c30-db11-9de9-000461428c89
fix MULTI_THREAD compilation of the new class

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6817 8aca7d54-2c30-db11-9de9-000461428c89
updated Makefile

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6818 8aca7d54-2c30-db11-9de9-000461428c89
misc. extra fixes/improvements related to viewport_t

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6819 8aca7d54-2c30-db11-9de9-000461428c89