Synchronization of group properties

[kittyfly]

This is a non-urgent issue that might be a problem later, so documenting it here.

Object group properties (the properties that are only meaningful in SOG, not in SOP), such as group position, better be only sync'ed for root prim in the group. That is, similar to physical properties that only exist in the RegionModule's sync cache for physical prims, group properties should only exist for root prims in the sync cache. For non-root prims, CurrentlySyncedProperties.ContainsKey(property) == false, if the property is a group property.

When synchronizing a group property, the sender only sends out a sync message for the root prim. Similarly, the receiver should only update the group property in the sync cache of the root prim.

[dslake]

In the current version, I think group properties are only sent for root prims but they are definitely stored in the cache for all prims which is unnecessary. I'll confirm this and track the issue of storing group properties for non-root prims. Thanks.