Calculating the clint area size

[Robert Roebling <roebling ruf uni-freiburg de>]

  Good day,

I need to calculate the size of a window, which is actually
visible on screen and not (partially) covered by borders.
scrollbars or any other areas I cannot draw on.

When using a simple widget, this can (I think) be achieved
by subtracting klass->xthickness from the actual size
of the widget.

But I also have a ScrolledWidget. This widget may have
scrollbars or may not have any. It contains a client area
(aka viewport) which can have the style sunken_in,
sunken_out or sunken_none.

How can I then calculate the size of the area, which I can
draw on?

     Thanks alot,

             Robert Roebling