bug#29279: Sharing the margins

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 29279@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 13 Nov 2017 21:02:23 +0200
> 
> On 11/13/17 8:15 PM, Eli Zaretskii wrote:
> 
> > No, to the left.  If a Lisp program wants to align to the right, it
> > should insert white space before the actual text.
> 
> That's only possible if it knows the resulting width. Which it will, in 
> the current state of the discussion.
> 
> Still, it seems unnecessary if the somewhat faster C code could 
> implement that for every user.

We could add that later if needed.  It's not rocket science, it's just
more complexity.

> > posn-at-point goes _once_ from window-start to the specified position,
> > so on average it traverses half a window, once.  By contrast, we are
> > now talking about redisplaying the window twice, and one of these 2
> > times must traverse the entire window.  So we are talking about
> > threefold slow-down on the average.
> 
> 3-fold slowdown from 500 FPS seems acceptable to me.

For each redisplay cycle?  On top of disabling most redisplay
optimizations?  I doubt that.