nondeterminism warning

lib/common/src/bvh.rs

@@ -112,6 +112,7 @@ impl<L> BvhNode<L>{
 			Ratio<Planar64,Planar64>:From<T>,
 			F:Fn(&L,&Ray)->Option<T>,
 	{
+		// source of nondeterminism when Aabb boundaries are coplanar
 		let mut nodes=BTreeMap::new();
 
 		let start_time=start_time.into();