Steam and smoke particle updates: particles now start off at one velocity and decay to a gentle upwards velocity before disappearing. Particles now also disappear only at the end of their duration, not the current duration (e.g. when throttling down a steam loco).
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@1946 a9f3cfff-6c41-7446-b0ca-052bd761e6eb