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

[Eli Zaretskii]

> From: Dmitrii Kuragin <kuragin@google.com>
> Date: Fri, 9 Sep 2022 08:48:57 -0700
> Cc: Eli Zaretskii <eliz@gnu.org>, 57434@debbugs.gnu.org
> 
> If everyone is OK with adding a new frame-local elisp flag, then I can 
> prepare a patch.

I think the conclusion in the message you quoted was that we will need
a function, not just a variable.  That's because we need to allow two
different protocols for this, and we should allow switching between
the protocols.