bug#14567: Scrolling of large images

[Eli Zaretskii]

> Date: Sat, 15 Jun 2013 14:26:13 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 14567@debbugs.gnu.org
> 
> > From: Thierry Volpiatto <thierry.volpiatto@gmail.com>
> > Date: Sat, 15 Jun 2013 13:13:05 +0200
> > 
> > however it would be great to be able to scroll horizontally too.
> 
> That's a much larger project, since we lack infrastructure to scroll
> horizontally by pixels.

Actually, I see that I was wrong: set-window-hscroll can do the job.
The only problem is to get C-f/C-b and <left>/<right> keys invoke that
function when appropriate.  Patches are welcome.