[vtkusers] update confusion

[Sven]

Hi,
I am totally confused by how update works.
It seems when I display a slice of an image volume in a TkImageViewerWidget
every time I go to a different slice the imaging chain executes again!
Sometimes, it doesn't execute at all and I receive a "Pixel .. not in memory
error! How can I have the chain execute one for the whole volume independent
on what's on the screen?

Thanks!
Sven