bug#67810: 29.1; fonts use synthetic bold on Linux / pgtk

[Tim Ruffing]

To reproduce:
   1. `emacs` -Q on Linux with pgtk enabled
   2. `M-x select-frame-font` and select a font that doesn't have a
      bold weight
   3. Observe that the buffer indicator (`*scratch*`) in the status
      line is in bold (using synthetic bold glyphs)

This is annoying in combination with symbol fonts such as Nerd fonts.
The bold versions of some symbols look strange, and worse, they may be
too wide to fit two glyphs and are clipped then.

What I have tried:
 * I thought this is a font-config thing. /etc/fonts/conf.d by default
   has a symlink 90-synthetic.conf that sets the "embolden" attribute
   for font queries that ask for a bold font but only regular is
   available. Removing that symlink does make a difference when I try
   `fc-match "MY-FONT:weight=bold" --verbose | grep bold`. With the
   symlink, fc sets the embolden attribute, and without the symlink, it
   doesn't. I can even see the difference in emacs' (Pango) font
   selection dialog. But emacs itself doesn't seem to care about this
   and still creates a synthesized bold font. 
 * Cairo is supposed to pick this attribute up [1] and act accordingly,
   but that doesn't work. I tried to add a printf("synthesize: %d\n",
   cairo_ft_font_face_get_synthesize(font_face)); to the current emacs
   git (75fd7550ed6cede6c9e8224f1f2d62637c43fdd4) in ftcrfont_open, and
   this always prints "0" (even with the symlink in place!). AFAIU this
   means that Cairo should never embolden fonts, but some reason
   there's something in emacs that does it.  
 * describe-font only shows the medium/regular variant.  


[1]
https://gitlab.freedesktop.org/cairo/cairo/-/blob/master/src/cairo-ft-font.c#L1928-1936

In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
cairo version 1.17.8)
System Description: Arch Linux

Configured using:
'configure --with-pgtk --with-native-compilation=aot --sysconfdir=/etc
--prefix=/usr --libexecdir=/usr/lib --with-tree-sitter
--localstatedir=/var --with-cairo --disable-build-details
--with-harfbuzz --with-libsystemd --with-modules 'CFLAGS=-march=x86-64
-mtune=generic -O2 -pipe -fno-plt -fexceptions -Wp,-D_FORTIFY_SOURCE=2
-Wformat -Werror=format-security -fstack-clash-protection
-fcf-protection -g
-ffile-prefix-map=/build/emacs/src=/usr/src/debug/emacs -flto=auto'
'LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto=auto'
'CXXFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions
-Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
-fstack-clash-protection -fcf-protection -Wp,-D_GLIBCXX_ASSERTIONS -g
-ffile-prefix-map=/build/emacs/src=/usr/src/debug/emacs -flto=auto''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY
PDUMPER PGTK PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XIM GTK3 ZLIB

Important settings:
value of $LC_COLLATE: de_DE.UTF-8
value of $LC_CTYPE: de_DE.UTF-8
value of $LC_MESSAGES: en_DK.UTF-8
value of $LC_MONETARY: de_DE.UTF-8
value of $LC_NUMERIC: de_DE.UTF-8
value of $LC_TIME: en_DK.UTF-8
value of $LANG: en_US.utf8
locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
tooltip-mode: t
global-eldoc-mode: t
eldoc-mode: t
show-paren-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
blink-cursor-mode: t
line-number-mode: t
indent-tabs-mode: t
transient-mark-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg
rfc6068
epg-config gnus-util text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-
loaddefs
comp comp-cstr warnings icons subr-x rx cl-seq cl-macs gv cl-extra
help-mode bytecomp byte-compile cl-lib sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils rmc iso-transl tooltip cconv
eldoc paren electric uniquify ediff-hook vc-hooks lisp-float-type
elisp-mode mwheel term/pgtk-win pgtk-win term/common-win pgtk-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer nadvice seq
simple cl-generic indonesian philippine cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite emoji-zwj charscript charprop case-table
epa-hook jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button
loaddefs theme-loaddefs faces cus-face macroexp files window
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget keymap hashtable-print-readable backquote threads
dbusbind
inotify dynamic-setting system-font-setting font-render-setting cairo
gtk pgtk lcms2 multi-tty make-network-process native-compile emacs)

Memory information:
((conses 16 77248 5503)
(symbols 48 7107 0)
(strings 32 19670 2185)
(string-bytes 1 602889)
(vectors 16 15723)
(vector-slots 8 329133 12738)
(floats 8 27 46)
(intervals 56 270 0)
(buffers 984 12)