Implement terrain merging in TerrainChannel.

Modify archiver to use terrain merging when loading oars.
This makes displacement AND rotation properly work on terrain when loading oars.
Especially useful when loading legacy region oars into large varregions.

OpenSim/Region/Framework/Interfaces/ITerrainChannel.cs

@@ -26,6 +26,7 @@
  */
 
 using OpenSim.Framework;
+using OpenMetaverse;
 
 namespace OpenSim.Region.Framework.Interfaces
 {
@@ -56,5 +57,7 @@ namespace OpenSim.Region.Framework.Interfaces
         ITerrainChannel MakeCopy();
         string SaveToXmlString();
         void LoadFromXmlString(string data);
+        // Merge some terrain into this channel
+        void Merge(ITerrainChannel newTerrain, Vector3 displacement, float radianRotation, Vector2 rotationDisplacement);
     }
 }