Region crossings

This section is incomplete


 * Every OpenSimulator frame (approximately every 89ms), every avatar is checked to see whether it has crossed into a neighbouring region (currently in ScenePresence.CheckForBorderCrossing.
 * This check is done by extrapolating the avatar's position in the next 100ms at its current heading and velocity
 * If the extrapolation shows the avatar moving into a new region, then IEntityTransferModule.Cross is triggered to initiate the cross process.
 * Cross calls ISimulationService.UpdateAgent on the receiving simulator with information about camera position, animations, attachments, etc.
 * This feeds through to Scene.IncomingUpdateChildAgent in the receving simulator's region.
 * The receiving simulator adds attachment information to it's own data model for that agent.