Cleanup: Alembic, simplify material assignment code

Refactor material assignment code such that:
- `build_mat_map()` just returns the built map (instead of relying on
  modifying a map passed as parameter),
- `LISTBASE_FOREACH` is used to loop over a `ListBase` (instead of a
  hand-crafted for-loop),
- just `return` when not enough material slots can be created (instead
  of setting a boolean to false, then doing some useless work, then
  checking the boolean),
- reorder some code for clarity, and
- rename `mat_map` to `matname_to_material` so that the semantics are
  clearer.

No functional changes.

0 files changed, 0 insertions, 0 deletions