Use 32bit image counter (will increase ground memory overhead slightly so far)

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7428 8aca7d54-2c30-db11-9de9-000461428c89
Do not checksum over uninitialised fields

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7429 8aca7d54-2c30-db11-9de9-000461428c89
x64 compiles with MSVC

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7430 8aca7d54-2c30-db11-9de9-000461428c89