Since vertex and face normals can be calculated separately, it simplifies
things to further separate the two caches. This makes it easier to use
`SharedCache` to avoid recalculating normals when copying meshes.
Sharing vertex normal caches with meshes with the same positions and
topology allows completely skipping recomputation as meshes are
copied. The effects are similar to e8f4010611, but normals are much
more expensive, so the benefit is larger.
In a simple test changing a large grid's generic attribute with geometry
nodes, I observed a performance improvement from 12 to 17 FPS.
Most real world situations will have smaller changes though.
Completely splitting face and vertex calculation is slightly slower
when face normals aren't already calculated, so I kept the option
to recalculate them together as well.
This simplifies investigating the changes in #105920 which resolve
non-determinism in the vertex normal calculation. If we can make the
topology map creation fast enough, that might allow simplifying this
code more in the future.
Pull Request: https://projects.blender.org/blender/blender/pulls/110479
383a145a19