bug#62237: 28.1 or higher: 24-bit true color breaks colours in Emacsen built without X under GNU Screen

[Sebastian Tennant]

Hello there,

Summary: Support for 24-bit true colour, added in Emacs 28.1, breaks
         colours in Emacsen built without X support and running under
         GNU Screen (v4.08.00).

Steps to reproduce:

 1. Build Emacs >= 28.1 without X support.  The two configurations I have
    tested are below:

     ./configure\                          ./configure\
       --prefix=[…]\                          --prefix=[…]\
       --enable-check-lisp-object-type\       --enable-check-lisp-object-type\
       --disable-acl\                         --disable-acl\
       --without-dbus\                        --without-all\
       --without-gconf\                       --without-x\
       --without-gif\                         --with-file-notification=yes\
       --without-gsettings\                   --with-gnutls\
       --without-jpeg\                        --with-gpm\
       --without-modules\                     --with-json\
       --without-png\                         --with-mailutils\
       --without-rsvg\                        --with-modules\
       --without-selinux\                     --with-native-compilation\
       --without-sound\                       --with-libsystemd\
       --without-tiff\                        --with-small-ja-dic\
       --without-x\                           --with-sqlite3\
       --without-xpm\                         --with-threads\
                                              --with-xml2\
                                              --with-zlib\

 2. Launch a terminal program (e.g. GNOME Terminal) and run GNU Screen
    (bypassing any .screenrc):

     $ touch foo
     $ screen -c foo

 3. Run Emacs with 24-bit true colour active and observe the broken colours:

     $ COLORTERM=truecolor ./src/emacs -Q

    Screenshot: https://download.sebyte.me/misc/truecolor-active.png

 4. Run Emacs with 24-bit true colour inactive and observe the correct colours:

     $ COLORTERM= ./src/emacs -Q

    Screenshot: https://download.sebyte.me/misc/truecolor-inactive.png