- add allowShared argument to DerivedMesh.convertToDispListMesh to allow returned

DLM to share data from DerivedMesh (reduces some copying/memory allocation)
 - added displistmesh_copyShared function to copy a DLM but not duplicate any
   internal data
 - changed crease drawing to use DerivedMesh functions... this means varying
   edge width style of creases had to go, I replaced by using varying color to
   show crease weight instead. Don't think this is a big loss since the subsurf
   result gives you a much better indication of the crease weight anyway.
 - bug fix in mirror modifier, didn't copy edge creases from editmesh correctly

source/blender/python/api2_2x/NMesh.c

@@ -2470,11 +2470,11 @@ static PyObject *M_NMesh_GetRawFromObject( PyObject * self, PyObject * args )
  		{
 			int needsFree;
 			DerivedMesh *dm = mesh_get_derived_final(ob, &needsFree);
-			DispListMesh *dlm = dm->convertToDispListMesh(dm);
+			DispListMesh *dlm = dm->convertToDispListMesh(dm, 1);
 			nmesh = new_NMesh_internal(ob->data, dlm );
+			displistmesh_free(dlm);
 			if (needsFree)
 				dm->release(dm);
-			displistmesh_free(dlm);
  		}
  		break;
  	default: