[vtkusers] gradient background direction

[Florian Bruckner]

hi, 

is it somehow possible to set the direction of the gradient background?
I would need this to make different renderers in a renderer-window
visible to the user. 

Adding a box around the viewports would be another possibility, but i
also didn't find an easy solution for this! (where no additional
renderers / actors / ... have to be added, which could make the
application slower)

thanks
FloB