The correction bbc6bb3468 was still wrong because there it was
disregarded that `vert_ctx_len` does not necessarily indicate merges in
the same polygon.
Therefore, it is not safe to rely on `vert_ctx_len` to count possible
new polygons.
NOTE: It might be worth preempting part of the
`weld_poly_split_recursive` logic to identify what the new polygons are
in advance. But this can be left for a future refactor.
495a6ec6cc