[vtkusers] Mapper/Renderwindow Bug

[John Biddiscombe]

If you have
polydata->DataSetMapper->Actor->Renderwindow
then you
delete renderwindow
delete actor
delete Mapper
(in that order).
The actor unregisters itself from the mapper, but the mapper has
this->LastWindow set to the renderwindow (now invalid) and an access 
violation happens in the mapper destructor when it execute the following code.

vtkOpenGLPolyDataMapper::~vtkOpenGLPolyDataMapper()
{
   if (this->LastWindow)
     {
     this->ReleaseGraphicsResources(this->LastWindow);
     }
}

I regret that I haven't got time to find a fix, so if anyone who knows the 
mapper internals better can oblige...

Thanks

John B