Two main user-facing benefits: only nodes with actually changed
values have undo steps pushed, meaning reduced memory usage
and undo runtime, and the mask attribute is removed when cleared,
reducing memory usage and theoretically improving performance
afterwards.
On the code side, the changes consist conceptually of expanding the
PBVH vertex iter macro for each PBVH type and simplifying the resulting
logic. This results in significantly more code; the idea is that in the
future abstractions to reduce duplication will become clearer. We just
want abstractions that relate closely to the actual data processing,
which is much simpler than the existing macro makes it look.
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