bug#16649: Right margin overflows on neighboring windows.

[martin rudalics]

> Sorry, the right fringe renders correctly, but the margin still overflows. 
Attached is a screenshot of magit-log, where this can be seen. Below is a test 
case.
>
> Evgeni
>
> (let ((ov (make-overlay (point-min) (point-max))))
>   (overlay-put ov
>                'before-string
>                (propertize "o" 'display
>                            (list '(margin right-margin)
>                                  (propertize
>                                   "Laetamur quis quam, aliqua nostrum."
>                                   'face '(:background "yellow")
>                                   ))))
>   (set-window-margins nil (car (window-margins)) 30))

OK.  Please try again.

martin