IndexMask::complement() is often used in geometry processing
algorithms when a selection needs to be inverted, mostly just in
curves code so far.
Instead of reusing `from_predicate` and lookup in the source mask,
scan the mask once, inserting segments between the original indices.
Theoretically this improves the performance from O(N*log(N)) to O(N).
But with the small constant offset of the former, the improvement is
generally just 3-4 times faster. However in some cases like empty
and full masks, the new code takes constant time.
Pull Request: https://projects.blender.org/blender/blender/pulls/108331
49b48209e7