This PR greatly improves the subsetting of Typst. This should lead to decreased PDF files in nearly all cases, even more so when a Typst document contains many fonts. For many big CFF fonts, the file sizes will even be reduced from 1MB+ to just a few KBs.

See the READMEs in typst/subsetter#1 for how we are currently testing the subsetter to ensure that it is correct. Apart from the tests there, I also created a PDF document using typst containing many different fonts and a subset of glyphs. You can find it here:

typst.pdf

I tested it with all major PDF viewers and they all seem to display it fine.

Given this test file and also the tests in subsetter (as well as fuzzing), I do feel pretty confident about the correctness of the subsetter and hopefully there shouldn't be any major issues, but in the end we will only find out by actually trying it and maybe encourage people to especially check their PDF output when releasing the first release candidate. Some manual testing beforehand by volunteers would also be highly appreciated.

Fixes #895.

Merging is blocked by typst/subsetter#1 and typst/svg2pdf#76.

P.S.: The reason I had to split glyph sets and color glyph sets is that it is possible that for a color font, we use both, some glyphs for color fonts and some for normal text (I encountered this issue when fixing a test case with emojis and NZW). However, when subsetting we don't want to subset the glyphs that are only used as part of color fonts (since they will already be written as a type 3 font), and we also can't share the CMAP between the two fonts. This is why they had to be separated.