Make FontSystem not self-referencing and update fontdb and rustybuzz

[geieredgar]

This makes the std implementation of FontSystem not self-referencing, which in turn allows defining a db_mut method for mutable access to the underlying fontdb::Database (requested in #75).

The high level changes are:

• Adds a new FontKey struct that stores the fontdb::ID and (if the swash feature is enabled) the swash key data (u32, swash::CacheKey).
  • A FontKey can be obtained via the new FontSystem::get_font_key(id).
  • FontKeys are cached instead of caching Fonts.
• FontSystem::get_font now takes a FontKey instead of fontdb::ID and creates a Font on the fly, reusing the key data.
• Removes FontMatches.
  • FontSystem::get_font_matches now returns a Arc<Vec<FontKey>> instead of Arc<FontMatches<'a>>.
  • FontFallbackIter borrows fontdb::Database and creates the font on the fly using Font::from_key(db, key).

[notgull]

Just a heads up: before version 0.13.0, lookups in fontdb take linear time. As part of this PR, if you're making breaking changes anyways, I would bump fontdb to 0.13.0 so that lookups take O(1) time.

[geieredgar]

Ok, I have updated the fontdb dependency. The family name displayed in error messages is now the name of the first family in fontdb::FaceInfo or post_script_name if families is empty.

[geieredgar]

I fixed the duplicate ttf-parser entries by bumping rustybuzz to version 0.7. Not sure I can/should do something about the failed license requirement.

[geieredgar]

I've added Zlib to the list of allowed licenses. I think there is no harm in that.

[jackpot51]

@geieredgar I merged this since CI tests were passing but it appears to have drastically reduced the performance of the editor-test example, so I reverted. Please see if you can replicate this issue.

[geieredgar]

@jackpot51 Yes, I could replicate this issue. The reason is that rustybuzz::Face::from_slice takes a lot of time. I wrongly assumed it would behave similar to ttf_parser::Face::parse in terms of performance, which is intended to be cheap and therefore can be executed on the fly, but that isn't the case. I will look into trying to make rustybuzz::Face::from_slice not as expensive and then compare the performance of my approach with the old approach. Sorry for wasting your time!

[jackpot51]

Thanks for looking into this. If there was a way to do what the swash Font does it would be ideal.