Instead of letting Stylo filter `@font-face` rules, handle this
filtering in Servo. It doesn't make sense that Stylo knows about what
fonts Servo supports. This also cleans up a bit the way that this is
handled, giving an entire stylesheet of rules to the font cache to
process instead of letting each layout thread walk the rules. This
brings more of the font-related code into the FontCacheThread itself.
This is the first step toward adding WOFF2 support and fixing various
web font related bugs.
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
