[vtkusers] Disabling middle mouse button for vtkImageTracerWidget

[David Doria]

> David,
> I think except for this line with overrides the pointer type for "self" and
>  keeps the functionality in vtkImageTracerWidget...
>
>  vtkImageTracerWidget* self = reinterpret_cast<vtkImageTracerWidget
>
> It looks to me like ProcessEvents() is the callback function used for all of
> the callback operations, and the way ProcessEvents() is coded it defeats sub
> classing. Hope that helps,

Hm, so I would have to implement the ProcessEvents() function and
copy/paste everything except the middle mouse button stuff? That seems
quite silly... Is there a reason that it is implemented like this
instead of allowing subclassing?

David