[vtkusers] updating file inputs wtih vtkImageReader

[Dean Inglis]

in a gui app, I am reading in square 2D images with a vtkImageReader.
There is a downstream image processing/display pipeline that
taking the output from the reader into several other filters/objects.
For each new image the extent may change.  I have noticed that
if I start with a smaller square image and then later
open a larger image, the image is pulled into the pipeline.
In the reverse case, if I start with a larger image and try
to read in a smaller one, the reader doesn't update...I still
have the larger image.  How do I force the extents to be 
changed in this case?

Dean