Sound update. Start moving the "listener" to have doppler effect for all environmental sounds (e.g. level crossings). Added handling of Ignore3D() statement. Modified volume calculation to better reflect MSTS. Env sounds can now use volume curves controlled by listener speed. Also reduce chance of running into Bug #1140853 and Bug #1095210 by fixing of env sounds never getting out of scope, resulting in faster overflow of the number of sound sources.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@1604 a9f3cfff-6c41-7446-b0ca-052bd761e6eb