bug#19395: 25.0.50; Setting left fringe to 0 messes up window-width

[Eli Zaretskii]

> From: Titus von der Malsburg <malsburg@posteo.de>
> Cc: 19395@debbugs.gnu.org
> Date: Wed, 17 Dec 2014 10:48:25 -0800
> 
> The problem is that there may not be a clean solution to that
> problem.  A buffer can contain text is several font sizes and as far as
> I know there is no notion of a default size for a buffer, or is there?

Yes, there is: the 'default' face.

> In my special case, I have the font size under control and
> `term-window-width' would be good enough.

But if we want this function to be more generally useful, it
shouldn't be limited to the frame's canonical character size, and
should at least take the face-remapping into account.  Bonus points
for accepting a face as an argument and using that face's font
dimensions.