bug#16649: Right margin overflows on neighboring windows.

[martin rudalics]

> Could you please tell what exactly is wrong with display in the above
> test case?  And what does "the margin overflows" mean?  Maybe I'm
> blind, but I don't see anything wrong when I eval the above code.

You have to do C-x 3 and enlarge one of the windows to see the effects.
There were plenty of them - mostly related to not returning the right
values via window box related functions.  I also now adjust the window
margins when a window gets to narrow because otherwise the redisplay
engine gets confused when inserting new text.  Still pretty toothless
when confronted with more drastic specifications but the best I came up
with so far.

martin