bug#57434: 28.1.91; Terminal Emacs Mac OS flickering.

[Eli Zaretskii]

> From: Dmitrii Kuragin <kuragin@google.com>
> Date: Wed, 31 Aug 2022 09:00:51 -0700
> Cc: Eli Zaretskii <eliz@gnu.org>, 57434@debbugs.gnu.org
> 
> Sorry, but seems like it helped in some configuration, but when I tried it on 
> my second monitor, it didn't work:
> https://youtu.be/IHzJ0QtuTgs
> 
> `baud-rate` improves the situation somehow, so that some portion of 
> flickering disappears, but the issue is
> still there and looks the same that insertion over writing causes the issue.

Given our inability to pinpoint the root cause of the problem, and the
unsolved mystery why on some macOS systems the problem is much more
prominent than on other macOS systems, I tend to introduce a variable
that users could set from Lisp to tell Emacs not to use the optimized
insert/delete-lines algorithm in scroll.c.  Unless some significant
new ideas or facts emerge in the next day or two, that is.