BVH optimize panics when only one shape contained in it

[tlaukkan]

I am getting this error when attempting to optimize with only one shape in the bvh:

attempt to subtract with overflow thread 'sphere_intersector::tests::test_ray_intersect' panicked at 'attempt to subtract with overflow', /Users/tlaukkan/.cargo/registry/src/github.com-1ecc6299db9ec823/bvh-0.3.2/src/bvh/optimization.rs:144:43 stack backtrace: 0: backtrace::backtrace::libunwind::trace at /Users/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86 1: backtrace::backtrace::trace_unsynchronized at /Users/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/mod.rs:66 2: std::sys_common::backtrace::_print_fmt at src/libstd/sys_common/backtrace.rs:78 3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt at src/libstd/sys_common/backtrace.rs:59 4: core::fmt::write at src/libcore/fmt/mod.rs:1063 5: std::io::Write::write_fmt at /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/libstd/io/mod.rs:1426 6: std::io::impls::<impl std::io::Write for alloc::boxed::Box<W>>::write_fmt at src/libstd/io/impls.rs:156 7: std::sys_common::backtrace::_print at src/libstd/sys_common/backtrace.rs:62 8: std::sys_common::backtrace::print at src/libstd/sys_common/backtrace.rs:49 9: std::panicking::default_hook::{{closure}} at src/libstd/panicking.rs:204 10: std::panicking::default_hook at src/libstd/panicking.rs:221 11: std::panicking::rust_panic_with_hook at src/libstd/panicking.rs:470 12: rust_begin_unwind at src/libstd/panicking.rs:378 13: std::panicking::begin_panic 14: std::panicking::begin_panic 15: bvh::bvh::optimization::<impl bvh::bvh::bvh::BVH>::optimize at /Users/tlaukkan/.cargo/registry/src/github.com-1ecc6299db9ec823/bvh-0.3.2/src/bvh/optimization.rs:144 16: intersect::sphere_intersector::SphereIntersector::optimize at src/sphere_intersector.rs:111 17: intersect::sphere_intersector::tests::test_ray_intersect at src/sphere_intersector.rs:192 18: intersect::sphere_intersector::tests::test_ray_intersect::{{closure}} at src/sphere_intersector.rs:151 19: core::ops::function::FnOnce::call_once at /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/libcore/ops/function.rs:232 20: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once at /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/liballoc/boxed.rs:1017 21: __rust_maybe_catch_panic at src/libpanic_unwind/lib.rs:86 22: std::panicking::try at /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/libstd/panicking.rs:281 23: std::panic::catch_unwind at /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/libstd/panic.rs:394 24: test::run_test_in_process at src/libtest/lib.rs:542 25: test::run_test::run_test_inner::{{closure}} at src/libtest/lib.rs:451 note: Some details are omitted, run with RUST_BACKTRACE=full for a verbose backtrace.