Allow vectorized vcalls of Mesh methods

[merlinND]

Description

Allows accessing the Mesh interface through a new MeshPtr type. This will allow users to access the Mesh interface through a new MeshPtr. Before, it was only accessible on the individual Mesh instances (no vectorized calls).

meshes = mi.MeshPtr(si.shape)
# Not strictly needed, the non-mesh pointers are automatically zeroed-out
active &= si.shape.is_mesh() 

n = meshes.face_normal(si.prim_index, active=active)

Requires a DrJit-side change to allow CallSupport<Mesh>: https://github.com/mitsuba-renderer/drjit/pull/287

Testing

Added a unit test that checks the results of the vcall on MeshPtr against calls on the individual mesh instances.

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
• [x] 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

[merlinND]

Thank you! I rebased and applied the fix.

[njroussel]

Looks good now. Thank you!