[vtkusers] Scene Linked in Multiple Windows

[Colin Doncaster]

    Doing more digging it seems weird that there really isn't any way of creating a
renderer, ren1, and adding it to multiple vtkRenderWindows.  I am not to sure what's steps
VTK goes through to evaluate each function to end with the data need for the vtkRenderer
but for each window you'd only really need the renderer to re-evaluate itself to use a
different camera specified by the viewport and a handle different culling parameters - data
mappers shouldn't need to change.  It appears that the vtkProp contains a PickTag
member, so if picked and that set I don't see why either different renderer's or a
different vtkRenderWindow wouldn't pick that up.

    Also - is using vtkAssembly objects the common way of handling heirarchal structures?
It seems that if you'd want to be able to dynamically add objects too and from a scene with
undefined parenting you'd need each node to have a vtkAssembly and a vtkActor as vtkActor
can't have anything parented to it.

    Cheers - Colin.