sitting avatars hidding by their position. This is not as SL that uses

avatar sitting on root prim. All cases have good and bad situations, this is
simpler. Only SOG position changes triggers checks, rotation alone will
not. ( as happens in other cases ) (UNTESTED, as usual )

OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs

@@ -701,6 +701,11 @@ namespace OpenSim.Region.Framework.Scenes
                         part.GroupPosition = val;
                 }
 
+                foreach (ScenePresence av in m_linkedAvatars)
+                {
+                    av.sitSOGmoved();
+                }
+
                 // now that position is changed tell it to scripts
                 if (triggerScriptEvent)
                 {