- Add code documentation.
- Use Span and references rather than pointers everywhere.
- Move all core normalization logic into
`BKE_defvert_normalize_lock_map()`, and have the other variants call
that. This keeps all the logic in one place, which will help make
future changes easier since they only need to be made in one place.
- Add unit tests for `BKE_defvert_normalize_lock_map()`.
- Refactor `vgroup_normalize_all()` to be clearer and avoid an
unnecessary `goto`.
- Make both `vgroup_normalize_all()` and `paint_weight_gradient_exec()`
only call into a single core vertex normalization function, rather
than branching into one of many.
No functional change intended.
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