Somewhat improve avatar region crossings by properly preserving velocity when avatar enters the new region.

This commit addresses the following issues were causing velocity to be set to 0 on the new region, disrupting flight in particular
* Full avatar updates contained no velocity information, which does appear to have some effect in testing.
* BulletSim was always setting the velocity to 0 for the new BSCharacter.  Now, physics engines take a velocity parameter when setting up characters so we can avoid this.
This patch applies to both Bullet and ODE.
When performing region cross, don't add the velocity to the new position in ETM.CrossAgentIntoNewRegionMain() since this has already been performed by SP.CheckForBorderCrossing()