etc/fonts/conf.d: Disable bitmap fonts for Firefox (pdf.js)

[pdf.js commit 53134c0c0bb2 ([api-minor] Use a local font or fallback on
an embedded one (if it exists) for non-embedded fonts (bug 1766039))][1]
[breaks][3] [font selection in PDF documents][3], when the font is not
embedded and bitmap fonts, like X.Org fonts, are available. Work around
it by disabling bitmap fonts for Mozilla Firefox.

Suggested-by: Calixte Denizet <cdenizet@mozilla.com>
Resolves: mariux64/bee-files#2933

Copy commit message from [proposed (and rejected) bee-files commit][4]:

> This works around a Mozilla Firefox regression from 114.0 to 115.0.
> Firefox was upgraded from 114.0 to 115.0 around July 5th, 2023, and
> until now we had two problem reports. It’s actually a [regression][1]
> [in][2] [pdf.js][3], used to display PDF files inside the browser.
> Evince or other PDF viewers do not have the problem.
>
> The problem is, that for PDF documents using Helvetica, but not
> embedding the font, pdf.js chooses the X.Org font, despite Fontconfig
> matching *Helvetica* to *Nimbus Sans L*:
>
>     $ fc-match Helvetica
>     n019003l.pfb: "Nimbus Sans L" "Regular"
>
> As downgrading Firefox from 115.0.2 to 114.0 is unsupported by Mozilla
> for profiles – a warning is shown, and you need to start it with
> `--allow-downgrade` – remove the likely unused Helvetica fonts confusing
> Firefox/pdf.js.

[1]: https://github.com/mozilla/pdf.js/commit/53134c0c0bb2d19244dd8c31aa4e8e6d01a657e4
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1845551
[3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1839860
[4]: mariux64/bee-files#2935

etc/fonts/conf.d/70-no-bitmaps-firefox.conf

@@ -0,0 +1,19 @@
+<?xml version='1.0'?>
+<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
+<fontconfig>
+  <description>Disable bitmap fonts for Firefox/pdf.js to work around https://bugzilla.mozilla.org/show_bug.cgi?id=1839860</description>
+
+  <match>
+    <test qual="all" name="prgname" target="pattern" compare="eq">
+      <string>firefox</string>
+    </test>
+
+   <selectfont>
+     <rejectfont>
+       <pattern>
+         <patelt name="scalable"><bool>false</bool></patelt>
+       </pattern>
+     </rejectfont>
+   </selectfont>
+  </match>
+</fontconfig>