bug#14704: 24.3.50; cl-lib breaks built-in Emacs version

[Eli Zaretskii <eliz@gnu.org>]

> From: Juanma Barranquero <lekktu@gmail.com>
> Date: Mon, 24 Jun 2013 22:30:38 +0200
> Cc: Dmitry Gutov <dgutov@yandex.ru>, Glenn Morris <rgm@gnu.org>, lunaryorn@gmail.com, 
> 	14704@debbugs.gnu.org
> 
> On Mon, Jun 24, 2013 at 10:25 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > We lack a lot of infrastructure for that, like access from Lisp to
> > data structures manipulated by the display engine.
> 
> What kind of API are you envisioning? Yes, I know that a designing a
> precise API is half the work, I'm asking more about the general idea.

First, a caveat: I didn't really put a lot of thinking into this.

That said, what I had in mind was an API that would return a detailed
description of the current glyph matrix for a window.  Another API
should return the position and description (block, bar, hollow, etc.)
of the window's cursor.

Another useful feature would be to be able to produce the full
sequence of calls to display-specific back-end APIs which were
involved in a given redisplay cycle.  This would obviously require
design of some kind of language to describe these sequences.

There will be probably more once we start on this road, but the above
should be a good starting point, IMO.