Pick the location on the terrain from a given ray.
It basically works by casting a pick ray against the bounding volumes of the TerrainQuad and its children, gathering all of the TerrainPatches hit (in distance order.) The triangles of each patch are then tested using the BresenhamYUpGridTracer to determine which triangles to test and in what order.
Works on the XZ plane, with positive Y as up.
Pick result on a terrain patch with the intersection on the bounding box of that terrain patch.