bug#43058: 27.1; Support for other colour font formats

[Peter Oliver]

As I understand it, there are four competing standards for colour in TFF fonts: 
CBDT (Google), COLR (Microsoft), SBIX (Apple) and SVG (W3C).

I have tried CBDT, COLR and SVG in Emacs 27.1 on Fedora 32, and only CBDT works 
for me.

Gnome on Fedora 32 supports both CBDT and COLR, suggesting that the underlying 
libraries have the necessary support for COLR.

CBDT is a bitmap format, whereas COLR is a vector format; it would be good to 
be able to support both.  Further, the existence of 
https://github.com/googlefonts/colr-gradients-spec implies that Google could be 
considering switching from CBDT to COLR, suggesting the possibility that the 
CBDT format could become obsolete.

The NEWS file for Emacs 27.1 says:
“Multicolor fonts such as "Noto Color Emoji" can be displayed on
Emacs configured with Cairo drawing and linked with cairo >= 1.16.0.”
I would submit a patch to change this to mention that only CBDT is currently 
supported, but I’m not sure if that’s true on, e.g., Windows or MacOS.


In GNU Emacs 27.1 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.24.21, 
cairo version 1.16.0)
 of 2020-08-20 built on buildvm-x86-24.iad2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.12008000
System Description: Fedora 32 (Workstation Edition)

Configured using:
 'configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no --with-xwidgets --with-modules --with-harfbuzz
 --with-cairo --with-json build_alias=x86_64-redhat-linux-gnu
 host_alias=x86_64-redhat-linux-gnu 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong
 -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic
 -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection'
 LDFLAGS=-Wl,-z,relro
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS XWIDGETS
LIBSYSTEMD JSON PDUMPER GMP

Important settings:
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

-- 
Peter Oliver