[vtkusers] Bug in vtkRenderer with BackingStoreOn when resizing window

[Vladan Bato]

In order to prevent this bug from being forgotten, I put it in the 
bug-tracker (as bug #385).

Since this was quite critical for me, I also fixed the bug and attached 
the patch to the above bug report.

While experimenting with the backing store, I noticed that on some cards 
it's quite slow. For example on a Matrox G550 under Linux it takes over
300ms for a 640x640 window.

By looking at the vtkRenderer code I saw that the backing store is kept 
in main memory and transferred to the video buffer at each render.

The question is: Would it be possible to store it in video memory 
instead? That way the transfer would be much faster.

I don't know OpenGL, so I don't know whether this is feasible or not.

-- 
Vladan Bato
vbato at ieee.org