Noto CJK font fix

fonts.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!--

    Problem: After installing the Noto fonts (on my Debian `stretch`
      system, with `apt install fonts-noto`), Japanese text in my
      browser ends up displayed in the "Medium" style of the font
      rather than the "Regular" style.  The "Medium" style is too
      heavy for normal text.

    Solution: Configure fontconfig to look specifically for Regular.

      This can't be done with just the `style` property in the
      pattern, because Medium has `style=Medium,Regular` and
      Regular just has `style=Regular`, and there doesn't seem to be
      a pattern that will prefer the latter over the former.

      So we use `weight`, but there's a bug in the font metadata
      where DemiLight has the same weight as Regular, which causes
      us to get DemiLight.  Fix that bug too.

    To use:

    1. Put this file in place at `/etc/fonts/conf.d/71-noto-fix.conf`.

    2. Run `sudo dpkg-reconfigure fontconfig` to rebuild the
       fontconfig cache.  (Otherwise the `target=pattern` rules will
       take effect, but not the `target=scan` rules.)

    3. (Optional) Run `fc-match "Noto Sans CJK JP"` and observe that
       you now get the Regular variant.

-->
<fontconfig>
    <match target="pattern">
        <test name="family" compare="contains">
            <string>Noto Sans CJK</string>
        </test>
        <edit name="weight">
            <const>regular</const>
        </edit>
    </match>

    <match target="pattern">
        <test name="family" compare="contains">
            <string>Noto Sans Mono CJK</string>
        </test>
        <edit name="weight">
            <const>regular</const>
        </edit>
    </match>

    <!-- Regular has weight 80.  For a corrected DemiLight weight, we
         pick 55, somewhat arbitrarily but because that's the
         "demilight" value mentioned in the fontconfig manual:
         https://www.freedesktop.org/software/fontconfig/fontconfig-user.html
    -->
    <match target="scan">
        <test name="family" compare="contains">
            <string>Noto Sans CJK</string>
        </test>
        <test name="style">
            <string>DemiLight</string>
        </test>
        <edit name="weight">
            <int>55</int>
        </edit>
    </match>

    <!-- Similarly Thin is at 50, same as Light. -->
    <match target="scan">
        <test name="family" compare="contains">
            <string>Noto Sans CJK</string>
        </test>
        <test name="style">
            <string>Thin</string>
        </test>
        <edit name="weight">
            <int>40</int>
        </edit>
    </match>
</fontconfig>