Constant canvas::raqote_backend::SHARED_FONT_CACHE

const SHARED_FONT_CACHE: LocalKey<RefCell<HashMap<FontIdentifier, Font>>>;

The shared font cache used by all canvases that render on a thread. It would be nicer to have a global cache, but it looks like font-kit uses a per-thread FreeType, so in order to ensure that fonts are particular to a thread we have to make our own cache thread local as well.