Don't prefer STIXGeneral over the default font (#9711)

* Don't prefer STIXGeneral over the default font

STIXGeneral contains some glyphs for non-LGC scripts, but often doesn't implement these scripts fully. We should always try the browser's default fonts, which are likely to look nicer and have broader script support, before falling back to STIXGeneral.

* Explain why STIXGeneral shouldn't have precedence

* Add a regression test

.github/workflows/cypress.yaml

@@ -92,6 +92,12 @@ jobs:
             #      # Run 4 instances in Parallel
             #      runner: [1, 2, 3, 4]
         steps:
+            - uses: tecolicom/actions-use-apt-tools@v1
+              with:
+                  # Our test suite includes some screenshot tests with unusual diacritics, which are
+                  # supposed to be covered by STIXGeneral.
+                  tools: fonts-stix
+
             - uses: actions/checkout@v3
               with:
                   # XXX: We're checking out untrusted code in a secure context