[✨ feature] Scene: allow bypassing ray intersection acceleration data structures

[merlinND]

Description

For very simple scenes, the cost of building, maintaining and using acceleration data structures can sometime be greater than the time savings. This PR enables skipping them entirely.

Testing

See test_accel_bypass.py.

Checklist

• [x] My code follows the style guidelines of this project
• [ ] My changes generate no new warnings
• [x] My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• [x] I have commented my code
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license