bug#38442: 27.0.50; segmentation fault switching to cairo

[Eli Zaretskii <eliz@gnu.org>]

> From: Juanma Barranquero <lekktu@gmail.com>
> Date: Mon, 2 Dec 2019 19:08:13 +0100
> Cc: rpluim@gmail.com, ola.nilsson@gmail.com, 38442@debbugs.gnu.org
> 
> > Every font that can use Uniscribe can also use HarfBuzz.  So this is
> > not a problem, and users have no reason to wish to use Uniscribe when
> > they restore their sessions.
> 
> That's an argument to remove support for Uniscribe. I imagine that, if we
> haven't done that, it's likely because there could be some problematic
> font that would behave better with Uniscribe than Harfbuzz, isn't it?

Something like that.  Basically, we didn't remove any backend on any
platform in Emacs 27 yet, as this is the first release which uses
HarfBuzz.  It's an insurance policy.

(Uniscribe in particular was deprecated by MS, so we will have to drop
it sooner or later, because at some point it will no longer come with
Windows.)

> All in all, I think I would prefer to keep font-backend and filter it out
> on restoring when/if necessary; but I don't have a compelling use case,
> so I'll just filter it out entirely. We can always change it later if the
> need arises.
> 
> I'll commit it ASAP.

Thanks.