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.