bug#32672: 27.0.50; image resize on window resizing

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 24 Sep 2018 14:25:15 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: juri@linkov.net, 32672@debbugs.gnu.org
> 
>  > I agree.  I think Lisp programs that use hooks provided by
>  > display-related code should generally expect to be called in many
>  > unrelated situations, and do whatever it takes by themselves to detect
>  > when it's "their" use case.  Expectations or requests for more focused
>  > hooks are impractical or even not feasible to implement, because core
>  > code knows very little about the Lisp application which uses the hook.
> 
> 'window-configuration-change-hook' is a great mess and is not
> display-related.

It is for the purposes of this discussion, since it's related to
changes in what windows display which buffers and which frames show
what windows.

> What users really need IMO is a single hook say
> 'window-state-change-functions' that we'd call in redisplay_internal
> in lieu of 'window-size-change-functions'.

redisplay_internal knows very little about changes in window
configurations, so I predict using this new hook will be as messy as
with the existing ones.

> We would run it if something in the state of a frame's root window
> changed (including size changes, changes of the windows' start
> positions and the selected window) and additionally provide a list
> of the differences in the frame's previous window state and the one
> redisplay is about to use.

We'd need to compute this list of changes first, and for that we will
need a whole slew of new variables and flags.  Currently, redisplay is
built on the opposite assumption: that each operation which could
potentially require redrawing some window(s) or frame(s) sets the
corresponding flags of the object that needs to be redrawn, without
any "explanation" of why that flag was set.