Terrain sample

This is an initial pass at a terrain query contribution for Kevin to look at.  You should be able to just use the button in the Terrain Sandcastle to test.  Note that visually it looks better in Columbus view as I think there might be a z clip render issue in 3d view.  

Hopefully the comments in the code will indicate what I'm trying to do, though I'm sure there's better ways of actually doing those things.