[vtkusers] downcasting in python wrappers in VTK 4.2.2

[Brad King]

> A quick confirmation.  Is this a good thing to do in general?  Are there
> issues with other Python modules.  I guess not but would appreciate
> clarification.

Tcl uses the flags by default, and we've never had a problem.  The
alternative is to go find every place in which we load a C++ module and
add a save/restore of the dlopenflags around it.

-Brad