Fixes issues in importers written in C++ (T100737):
- Materials had one reference count too much. Affected Collada,
Alembic, USD, OBJ importers, looks like "since forever".
- Active material index was not properly set on imported meshes.
Regression since 3.3 (D15145). Affected Alembic, USD, OBJ. Note:
now it sets the first material as the active one, whereas
previously the last one was set as active. First one sounds more
"intuitive" to me.
Reviewed By: Bastien Montagne
Differential Revision: https://developer.blender.org/D15831
f366d197db