Isolate vcalls per variant

[merlinND]

Description

Second attempt at solving the issue described in #1362, after discussion with @wjakob and @njroussel.

Requires mitsuba-renderer/drjit#313 on the drjit side.

Testing

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• 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
• I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license

[merlinND]

Hi @wjakob and @njroussel,

After discussion with @wjakob, we decided to remove the scope (int) component from the registry key, because we have not found a safe & convenient way to use it yet.
scope was only intended for performance improvements though, the current PR is sufficient to provide safety by isolating objects created under different Mitsuba variants.

Could you please have a look at the three related PRs:

1. Call: allow isolation per (variant, domain, scope) drjit-core#109
2. Call domain isolation per (variant, domain, scope) drjit#313
3. This one

If it looks good to you, I will rebase and cleanup the history before merging.