Writing to a bitmap from multiple threads causes races when writing to
bits within the same integer. Instead, write to a separate boolean array
while subdividing, and move that to the final mesh bit vector after.
Notes:
- The final copy to the bit vector could be replaced by a generic
`copy_from(Span<bool>)` call in the future.
- Theoretically we could entirely replace the `BitVector` with an
`Array<bool>`, but 1/8 the memory use for edges is likely worth it.
Hans Goudey