Previously, attribute propagation did not work correctly in when only
deleting edges and faces (but not points). Face and face corner attributes
were propagated wrongly or not at all respectively.
In order to keep the patch relatively small for the release branch,
it does not include some small optimizations. These can be done in 3.1:
* Use a `Span<int>` instead of `IndexMask` to avoid creating an
unnecessary `Vector<int64_t>`.
* Only prepare index mappings when there are actually attributes to
propagate.
Differential Revision: https://developer.blender.org/D13338
cf266ecaa6