Add an off-thread preparation step for downloaded vector fonts so
WOFF2 resources can be decompressed before LibWeb tries to create a
typeface from them. This avoids doing the conversion work on the main
thread during @font-face loading.
Expose raw WOFF2-to-TTF conversion from LibGfx's WOFF2 loader and use
that from the new preparation path. Keeping the libwoff2 integration
in LibGfx preserves the layering between LibWeb and the third-party
decoder while still letting LibWeb schedule the work off-thread.
