Instead of creating a new mesh from scratch, modify an existing mesh.
This allows us to keep derived caches for triangulation and bounds
alive more easily, and allows keeping materials and non-generic
attributes like vertex groups alive on the mesh.

It also has other performance benefits, since face and face corner
attributes aren't affected at all, and because of reduced overhead
from not allocating a new mesh.

Updating edge attributes is a bit more complicated now, since we
have to completely replace the arrays but keep the existing attribute
IDs around. The new mesh update tag is also slightly too specific IMO.
But I think both of those things will improve in the future because
of existing plans for further refactoring these areas:

• New attribute storage that gives pointer stability
• Further use and granularity of mesh update tagging that will make the correct API more clear

Fixes T102711