Removing either shape from a 2-shape BVH panics

[finnbear]

The following code panics but shouldn't:

    #[test]
    fn test_traverse_subtle_bvh() {
        let mut boxes = vec![
            UnitBox::new(0, TPoint3::new(0.0, 0.0, 0.0)),
            UnitBox::new(1, TPoint3::new(5.0, 5.0, 5.0)),
        ];
        let mut bvh = Bvh::build(&mut boxes);
        bvh.remove_shape(&mut boxes, 0, true); // removing 1 also panics
    }

thread 'bvh::distance_traverse::tests::test_traverse_subtle_bvh' panicked at src/bvh/optimization.rs:250:17:
Circular node that wasn't root parent=0 node=1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/std/src/panicking.rs:681:5
   1: core::panicking::panic_fmt
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/core/src/panicking.rs:75:14
   2: bvh::bvh::optimization::<impl bvh::bvh::bvh_impl::Bvh<T,_>>::remove_shape
             at ./src/bvh/optimization.rs:250:17
   3: bvh::bvh::distance_traverse::tests::test_traverse_subtle_bvh
             at ./src/bvh/distance_traverse.rs:386:9
   4: bvh::bvh::distance_traverse::tests::test_traverse_subtle_bvh::{{closure}}
             at ./src/bvh/distance_traverse.rs:380:34
   5: core::ops::function::FnOnce::call_once
             at /home/finnb/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/core/src/ops/function.rs:250:5