Avoid storing redundant and unnecessary data in temporary arrays during
face corner normal calculation with custom normals. Previously we used
96 bytes per normal space (`MLoopNorSpace` (64), `MLoopNorSpace *` (8),
`LinkData` (24)), now we use 36 (`CornerNormalSpace` (32), `int` (4)).
This is achieved with a few changes:
- Avoid sharing the data storage with the BMesh implementation
- Use indices to refer to normal fan spaces rather than pointers
- Only calculate indices of all corners in each fan when necessary
- Don't duplicate automatic normal in space storage
- Avoid storing redundant flags in space struct
Reducing memory usage gives a significant performance improvement in
my test files, which is consistent with findings from previous commits
(see 9fcfba4aae). In my test, the time used to calculate
normals for a character model with 196 thousand faces reduced from
20.2 ms to 14.0 ms, a 44% improvement.
Edit mode isn't affected by this change.
A note about the `reverse_index_array` function added here: this is the
same as the mesh mapping functions (for example for mapping from
vertices to face corners). I'm planning on working this area and hoping
to generalize/reuse the implementation in the near future.
Pull Request: https://projects.blender.org/blender/blender/pulls/107592
17d161f565