bug#16649: Right margin overflows on neighboring windows.

[martin rudalics]

>>From emacs -Q, with version from trunk.
>
> STEPS:
>
> (set-window-margins nil (car (window-margins)) 30))
> C-x 3
>
> Resize, until the right window is smaller than the margin.
>
> RESULT:
>
> The right window's right fringe will be drawn on top of the left window's 
right margin.

Should be fixed now.  Though using such large margins surely means
asking for trouble.

Thanks, martin