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.
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