BulletSim: zero velocity when avatar not moving.

This fixes a movement jitter that happens when an avatar is standing on a
tilted surface.

OpenSim/Region/Physics/BulletSPlugin/BSActorAvatarMove.cs

@@ -278,6 +278,7 @@ public class BSActorAvatarMove : BSActor
         if (m_controllingPrim.IsStationary)
         {
             entprop.Position = m_controllingPrim.RawPosition;
+            entprop.Velocity = OMV.Vector3.Zero;
             m_physicsScene.PE.SetTranslation(m_controllingPrim.PhysBody, entprop.Position, entprop.Rotation);
         }