This commit adds support for computing the codepoint coverage for fonts
loaded from font-dirs and the built-in, in-memory fonts.
What this means is that if you have eg: a font with chinese glyphs in
your font-dirs but not explicitly listed in your wezterm config, if
chinese text is rendered and no match from your config is found, wezterm
will be able to find the font from your font-dirs and use that
implicitly.
Computing the codepoint coverage is relatively expensive so we defer it
until we need to perform it, and cache it.
Record a notion of the state of the invalidations that we've sent
to the client so that we can skip sending updates if nothing has
changed since the last push.
