By the way, this wouldn't fix #91. There are still outstanding issues with font loading that still slow it down in certain cases.

@notgull Could you check if this improves the load times? I've added a commit so it should only load the fonts that are really needed/used.

@notgull Could you check if this improves the load times? I've added a commit so it should only load the fonts that are really needed/used.

This does appear to improve the loading time on my system. You can notice main wait a moment to load the fonts. With this branch it does still take a tiny bit but it's generally done when Plasma's pop in animations for new windows finish.

Making FontSystem::get_font take &mut self also allows us to lazily call make_shared_face_data.

User question: I'm upgrading to cosmic-text 0.10 in an internal crate, and have a loop of this form:

for face in system.db().faces() {
    let font = system.get_font(face.id).unwrap();
    let font_ref = font.as_swash();

This won't compile because the .faces() iterator holds a borrow on system, while get_font() also wants to mutably borrow system. Is there a better solution than precomputing the list of IDs with let face_ids = system.db().faces().map(|face| face.id).collect::<Vec<_>>();?