Material slots: on adding a new one, the material datablock is now copied again

like in 2.4x. This can result in material datablocks you don't need, but not
doing it seems to cause too much confusion.

source/blender/blenkernel/intern/material.c

@@ -786,6 +786,13 @@ int object_add_material_slot(Object *ob)
 	
 	ma= give_current_material(ob, ob->actcol);
 
+    if(ma == NULL)
+		ma= add_material("Material");
+	else
+		ma= copy_material(ma);
+
+	id_us_min(&ma->id);
+
 	assign_material(ob, ma, ob->totcol+1);
 	ob->actcol= ob->totcol;
 	return TRUE;