bug#32672: 27.0.50; image resize on window resizing

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 24 Sep 2018 19:37:08 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: juri@linkov.net, 32672@debbugs.gnu.org
> 
> As I said, redisplay would not have to care about that at all.  It
> would simply call 'window-state-change-functions' where it calls
> 'window-size-change-functions' now.  And running
> 'window-state-change-functions' would use one boolean set (among
> others) instead of where 'run-window-configuration-change-hook' gets
> called now and which it resets.  Iff that boolean was set, it would
> start to find all windows where a relevant change occurred and run the
> functions.  Buffer-locally iff a window shows the buffer for which the
> local hook was set and something changed for that window.

Those functions will need to keep track of the changes, or record the
previous state attributes somewhere, to do their job, right?  Are you
saying that these are already recorded/tracked?  If not, they will
need to be added, which was the point I was making.

> The great advantage for users and application programmers would be
> that their functions would run once only and only if something really
> changed since last redisplay.

Even the "once" part might be problematic, because redisplay_internal
sometimes re-runs its code more than once, as you know.  The "really
changed since last redisplay" is even trickier, because a given
redisplay cycle doesn't always finish all of its job, it can stop in
the middle and return after doing only part of what needs to be done,
in which case some of the windows are not updated, and we will have
lost our point of reference.