* Use cosmic_text `metadata` attr to write down the `FontId` from the
input run to avoid searching the list of fonts when laying out every
glyph.

* Instead of checking on every glyph if `postscript_name` is an emoji
font, just store `is_known_emoji_font`.

* Clarify why `font_id_for_cosmic_id` is used, and when its use is
valid.

Release Notes:

- N/A