[vtkusers] Valid method in merging Actors needed

[徐志華]

I had a vtkActorCollection to be merge to one vtkActor, for the efficiencies 
of rotaing.
there is one method in class vtkActor->GetActors,
the document about method says that it's used for what I mean.
but , the outlet seems not the thing, the official method did not carry out 
what it meant, it only return an empty Actor.
Is there any one help me out the valid method in merging Actors?