BulletSim: add logic to turn off pre-step actions when object goes
non-active. This turns off 'setForce', 'setTorque' and 'moveToTarget'
when the object is selected or made non-physical.
BulletSim: reduce jitter in avatar velocity when walking or flying.
OpenSimulator is VERY sensitive to changes in avatar velocity and
will send an avatar update message when velocity changes more than
0.001m/s. This significantly reduces the number of avatar update messages by
smoothing the avatar velocity returned by Bullet.