Fixed weapon particle spews not taking launch velocity properly into account if $Acceleration Time is used.

git-svn-id: svn://svn.icculus.org/fs2open/branches/zookeeper@11085 387891d4-d844-0410-90c0-e4c51a9137d3
Trunk sync r11085.

git-svn-id: svn://svn.icculus.org/fs2open/branches/zookeeper@11086 387891d4-d844-0410-90c0-e4c51a9137d3