// TODO: it seems to me that the DocumentBuilder should track the "IMesh" as it is added.
// However I have to take into account the fact that the DocumentBUilder is aware of GeometryBuilder but not IMesh
public static int AddNode(this DocumentBuilder db, Matrix4x4 transform, IMesh mesh, IndexedSet <IMesh> meshes)
{
var gid = mesh != null?meshes.GetOrAdd(mesh) : -1;
db.AddNode(transform, gid, -1, -1);
return(db.Nodes.Count - 1);
}