Fix Scene datablocks being created with a real user while never having any real datablock user.

Now using new system dedicated to that kind of cases, id_ensure_real_user(), instead.
That way, usercount of Scenes is handled correctly at deletion time.

Reported by @sergey over IRC, thanks.

source/blender/blenkernel/intern/scene.c

@@ -819,6 +819,8 @@ Scene *BKE_scene_add(Main *bmain, const char *name)
 	Scene *sce;
 
 	sce = BKE_libblock_alloc(bmain, ID_SCE, name);
+	id_us_min(&sce->id);
+	id_us_ensure_real(&sce->id);
 
 	BKE_scene_init(sce);