Fix issue where avatar and script chat could sometimes be heard from anywhere in neighbouring regions.

This was due to a silent uint overflow in ScenePresence.UpdateChildAgent() corrupting child agent positions
when the child agent was in a region with a greater x or y map co-ord than the root agent region
Probably introduced in beeec1c.
This still will not function properly with very high region map co-ords (in the millions) but other parts of the code don't handle this properly anyway.
Looks to address http://opensimulator.org/mantis/view.php?id=7163