Add Terrain CopyRegion method

[emiletrep]

We compile our maps including our terrain to a model file using TerrainRegion instances. It would be great to be able to generate that TerrainRegion through our remodel script. We also need Terrain.MaxExtents for it to work.

Thank you :)

[jeparlefrancais]

I don't know how hard this can be but I can give it a try!