Optimise Tlas building

[Vecvec]

Connections
First step on #9946

Description
Some low hanging fruit optimisations which save a few miliseconds (total ~7 ms in an initially 17-19 ms function)

Testing
existing tests should cover this

Squash or Rebase?

Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[kpreid]

I don’t know much about ray tracing or this part of the code, so here are a couple of comments aimed at improving the comprehensibility of the code.