bug#46827: Broken initial size of GTK3 frame

[Eli Zaretskii]

> From: martin rudalics <rudalics@gmx.at>
> Date: Tue, 2 Mar 2021 10:07:02 +0100
> Cc: 46827@debbugs.gnu.org
> 
>  > So the problem still seems that redisplay misses a pending
>  > window change.
> 
> Confirmed meanwhile.  The attached patch fixes the problem here.  Eli
> will tell use whether we can put that do_pending_window_change in some
> suitable safe place within redisplay_internal.

What's wrong with this place?  But please note that after calling
do_pending_window_change you need to perform the "maybe the selected
window changed" dance we do after other similar calls.

Otherwise, I'm okay with this change, but only on master.  Emacs 27.2
will have to make do with what we have now.