Fix T100737: OBJ/USD import: imported object has no active material, material has 2 users

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

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc b/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
index a570b374231..6b19d6573af 100644
--- a/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
+++ b/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
@@ -296,6 +296,8 @@ static Material *get_or_create_material(Main *bmain,
   const MTLMaterial &mtl = *materials.lookup_or_add(name, std::make_unique<MTLMaterial>());
 
   Material *mat = BKE_material_add(bmain, name.c_str());
+  id_us_min(&mat->id);
+
   ShaderNodetreeWrap mat_wrap{bmain, mtl, mat, relative_paths};
   mat->use_nodes = true;
   mat->nodetree = mat_wrap.get_nodetree();
@@ -319,6 +321,9 @@ void MeshFromGeometry::create_materials(Main *bmain,
     }
     BKE_object_material_assign_single_obdata(bmain, obj, mat, obj->totcol + 1);
   }
+  if (obj->totcol > 0) {
+    obj->actcol = 1;
+  }
 }
 
 void MeshFromGeometry::create_normals(Mesh *mesh)