[vtkusers] Visible-surface determination

[Simon Faust]

Hi,
I use vtkTkRenderWidget in order to display several objects that 
alltogether form a building with some installations in it. Problem ist 
that objects that should not be visible (i.e. the floor of a room when 
your viewpoint is over the roof) "jump" into the foreground and overlay 
objects that should be visible. The error occurs when rotating or 
zooming around/in the scene. Any hints welcome.
Simon