Here is a list of things to change :

The zoom and subsample factors are imtegers. It would be neat to
use floats instead, plus some kind of interpolation.

When a block is put in an image, the whole image is renormalized,
but DitherInstance only updates the corresponding block. Solution:
either change the DitherInstance code or add a refresh command.

Compile visu with the TIX extension and use their nice file selection
box.

Disable OPEN button in visu.tcl when no images are displayed.

Provide a mecanism to read a file argument, display the image, and 
yet use the command line.

Finish the getline option, and finish the default.tcl script to display
profiles/histograms.

Fix problem when images are deleted -> grab the events and forbid
interaction with other windows.



