vtkWeakPtr

There is now a vtkWeakPtr<T>, a weak reference to vtkObjectBase instances, which is thread-safe to management of the underlying object on other threads (versus vtkWeakPointer<T> which is not thread-safe at all).

Note that this is a separate class because vtkWeakPointer’s API cannot be supported in a thread-safe way. Instead of stuffing a thread-safe implementation inside of it while also allowing the thread-unaware API available, this new class is available to make sure that all uses are thread-aware.