(irtviewer.l) : Resize viewport when irtviewer is resized.

Fix viewport resizing. 

Before this PR, ``draw-line``, ``draw-arrow``, and ``draw-circle`` methods do not work because viewport size is not changed.
```
(make-irtviewer)
;; work correctly
(send *irtviewer* :draw-objects)
(send *irtviewer* :viewer :draw-line #f(0 0 0) #f(100 100 100))
(send (make-coords) :draw-on :flush t)

;; does not  work correctly
(send *irtviewer* :resize 700 700)
(send *irtviewer* :draw-objects)
(send *irtviewer* :viewer :draw-line #f(0 0 0) #f(100 100 100))
(send (make-coords) :draw-on :flush t)
```

After this PR, above two examples work correctly.