bug#15420: 24.3; Symbols like 🚴 (U+1F6B4) are not displayed by default

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: Mike FABIAN <maiku.fabian@gmail.com>,  larsi@gnus.org,
>   15420@debbugs.gnu.org
> Date: Thu, 15 Aug 2019 17:19:55 +0200
> 
> > You will see in lisp/international/fontset.el that the #x1F680 --
> > #x1F6FF block is now configured to use Symbola by default, which is
> > why the BICYCLE and BICYCLIST are displayed without any changes.  As
> > for the Egyptian Hieroglyphs, we don't do anything about them,
> 
> Is this for technical reasons (e.g. redisplay speed) or a policy
> decision such as not enough free fonts?  I ask because e.g. Firefox
> seems to automatically find an installed font; for example, on my system
> Firefox displays the hieroglyphs on
> <https://en.wikipedia.org/wiki/Egyptian_Hieroglyphs_(Unicode_block)>,
> presumably using the Aegyptus fonts installed in my system, while EWW
> does not display the hieroglyphs (but does e.g. display the emoticons on
> <https://en.wikipedia.org/wiki/Emoticons_(Unicode_block)>, some using
> DejaVu, some using Symbola).

Firefox has a long list of known fonts stored in its database out of
the box.  We don't for 2 reasons: (1) we don't want to reference
non-free fonts, and (2) we want to be able to support the use case of
installing a font while the session runs.

The above may or may not be relevant to the Egyptian Hieroglyphs case;
volunteers are welcome to step through the font lookup process in
Emacs in that case and see whether we should add something to our
fontset-related databases.  After all, this script is quite rare, and
was added to Unicode at a relatively late date, so something might be
amiss.