Blender Git Loki

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.