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/alembic/intern/abc_reader_mesh.cc b/source/blender/io/alembic/intern/abc_reader_mesh.cc
index bacc1f06599..5b68b80b181 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.cc
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.cc
@@ -26,6 +26,7 @@
 #include "BLI_math_geom.h"
 
 #include "BKE_attribute.h"
+#include "BKE_lib_id.h"
 #include "BKE_main.h"
 #include "BKE_material.h"
 #include "BKE_mesh.h"
@@ -93,6 +94,7 @@ static void assign_materials(Main *bmain,
     mat_iter = matname_to_material.find(mat_name);
     if (mat_iter == matname_to_material.end()) {
       assigned_mat = BKE_material_add(bmain, mat_name.c_str());
+      id_us_min(&assigned_mat->id);
       matname_to_material[mat_name] = assigned_mat;
     }
     else {
@@ -101,6 +103,9 @@ static void assign_materials(Main *bmain,
 
     BKE_object_material_assign_single_obdata(bmain, ob, assigned_mat, mat_index);
   }
+  if (ob->totcol > 0) {
+    ob->actcol = 1;
+  }
 }
 
 } /* namespace utils */