bug#30699: 26.0.91; buffer contents flicker on macOS frames when frames are resized

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Mon, 5 Mar 2018 08:21:45 -0800
> Cc: Alan Third <alan@idiocy.org>, 30699@debbugs.gnu.org
> 
> > AFAIK, no.  Redrawing a garbaged frame involves clearing it and
> > completely redrawing all of its windows, and that is what you perceive
> > as flicker.
> 
> Thanks, I was thinking about double buffering--painting both the clear
> and the next paint on the same buffer before swapping it in, but I'm
> sure that's much more simplistic than what is actually happening.

We already have double-buffering on X, perhaps it can be implemented
on macOS as well.