bug#31702: 26.1; Effective font height is different from set value

[Carlos Piat <carlosjosepita@gmail.com>]

I'm setting the Hack Mono font for the default face with size 110:

  (set-face-attribute 'default nil :font "Hack" :height 110)

but I noticed that the result is slightly different from what
gnome-terminal renders for "Hack 11". Namely, in emacs the font looks
bigger. So I checked the attribute value in the customize-face
screen for the default face, which showed a height of 113.

Playing a bit with different heights I realized that the number I set is
never exactly the setting that ends up being effective, as if a rounding
error or something was happening somewhere.

Even if I set the font as "Hack 10" or set it from the standard dialog
that is available in the menu bar the result is the same.

Some relevant libraries in my system are:

  local/freetype2 2.9.1-1
    Font rasterization library
    
  local/libxft 2.3.2-1
    FreeType-based font drawing library for X

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-05-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:	Manjaro Linux

Recent messages:
nil
Creating customization items...
Creating customization items ...done
Resetting customization items...done
Creating customization setup...done
nil
Entering debugger...
previous-line: Beginning of buffer
nil
Auto-saving...

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  show-paren-mode: t
  diff-auto-refine-mode: t
  pyvenv-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils cl-print debug crm vc-git base16-default-dark-theme
base16-theme paren cl-extra yasnippet elec-pair highlight-indentation
flymake-proc flymake warnings company edmacro kmacro pcase help-fns
radix-tree help-mode elpy find-file-in-project ivy delsel ivy-overlay
ffap thingatpt windmove diff-mode easy-mmode elpy-shell pyvenv esh-var
esh-io esh-cmd esh-opt esh-ext esh-proc esh-arg esh-groups eshell
esh-module esh-mode esh-util elpy-profile elpy-django s elpy-refactor
subr-x python tramp-sh tramp tramp-compat tramp-loaddefs trampver
ucs-normalize shell pcomplete parse-time format-spec advice json map ido
grep compile comint ansi-color files-x etags xref project ring cus-edit
cus-start cus-load wid-edit finder-inf info package easymenu epg-config
url-handlers url-parse auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache url-vars seq byte-opt gv bytecomp
byte-compile cconv cl-loaddefs cl-lib time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win
x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic 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 charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 364347 15038)
 (symbols 48 32967 1)
 (miscs 40 772 758)
 (strings 32 72107 1797)
 (string-bytes 1 2082553)
 (vectors 16 52862)
 (vector-slots 8 929416 10894)
 (floats 8 121 290)
 (intervals 56 576 99)
 (buffers 992 15))