Duke3D: Ensure player maintains velocity when going through a portal.

Best place to test this is the E1L1 ventillation shaft, especially with inittimer set low, such as 30Hz or something. You'll see the player actor comes to a stop mid-way for one game tic before continuing to free-fall.

• The current setup that's been long-standing has the player come to a complete stand-still for one tic while transiting through a portal.
• By saving off the velocity difference between the player's pos/opos, we can re-apply that to opos to make the portal transition truly seamless.