Now PhysicsActor.Velocity.set and PhysicsActor.SetMomentum do the same thing
of setting the instantanious avatar velocity. PhysicsActor.TargetVelocity
sets a velocity target and the movement motor is used to accelerate the'
avatar to that velocity.
confusion about push forces. The latter is an external, physics engine interface
feature (the force parameter has a different unit if pushing vs adding force)
and that distinction is not used internally.
that works for all physics engines.
BulletSim: implement an optimized version of SetVehicle() that still
uses the vehicle code to do bounds checking but does it in one taint
operation.
