Fix Bvh::optimize_incremental non-determinism

[raysplaceinspace]

Closes #402

There is a non-determinism issue in Bvh::optimize_incremental when you are restoring your Bvh from a serialized snapshot. It occurs because Bvh::optimize_incremental relies on significant state in BvhWorkspace which is ephemeral and cannot be serialized. This change moves the significant fields into Bvh so that they do get serialized, enabling deterministic execution once again.

I've written a lot more background on these two tickets:

• Non-determinism in Bvh::optimize_incremental when restoring from serialized snapshot #402
• Non-determinism in BroadPhase when restoring from serialized snapshot rapier#910

I have verified that all tests pass, and I have verified that this fixes the non-determinism issue in my application.

I am happy to rework this in other ways if you would prefer.