[vtkusers] Force uniform scaling in a vtkBoxWidget

[Serge Lalonde]

Hello,

I would like to enforce a uniform scaling in the 
vtkBoxWidget's.InteractionEvent. I have most of the code working but I 
would ideally like to detect if the operation is really a scaling 
because it may be moving or rotating instead (in which case I do the 
default).

The problem is that there is no API that I could find in vtkBoxWidget 
that can tell me this. The frustrating part is that there is a private 
WidgetState enum, but it's protected.

One idea was to compare the vtkBoxWidget's transform with its previous 
state and detect the biggest difference in orientation, position and 
scale (since only one can change), but that seems error prone.

I could modify the code and add a vtkSetMacro()/vtkGetMacro() for the 
State field, but before I do that I wanted to check if there was a 
better way to do it. If I resort to this source code change, any chance 
that this can become a "standard" feature?

Thanks.