When encountering a @font-face rule, StyleComputer will now fire off
a resource request and download the first source URL specified.
Once downloaded, we try to parse it as a TrueType font file, and if it
works, it's added to a cache in StyleComputer. This effectively makes
fonts per-document since every document has its own StyleComputer.
This is very unoptimized and could definitely use some caching, etc.
But it does work on Acid3. :^)