Resolves #4582
…just by deleting improve_glyph_sets!

There are two sources of information for /ToUnicode: the glyph_set recorded while write_text, and cmap tables of the font.
improve_glyph_sets leverages the font. It was refactored into a function in #4154, but the real code even predates ed6550f (2 years ago).

improve_glyph_sets was necessary before ad34763, when a glyph_set was a list of glyphs, and we had to search the font (again) for their texts. (Each glyph represents a text, which is a Unicode code point or a sequence of code points (e.g. ligature).)

In ad34763, the glyph_set is refactored to a map from glyphs to texts.
Now we have enough information for /ToUnicode CMap—no need to search the font.

Changes

If the glyph…

• …represents a single character…
  • …and is mapped from only one code point:
    No change.
  • …and is shared by multiple code points (e.g. CJK unified/compatibility):
    /ToUnicode changes from the largest code points to the first occurrence, and fixes /ToUnicode in PDF can be wrong if a glyph is mapped from multiple code points #4582.
• …represents a sequence of characters (e.g. ligature)…
  • …and they are also encoded as a single code point for compatibility (e.g “fi”/fi):
    /ToUnicode changes from a single compatibility code point (fi) to the sequence (fi).
    The behaviour in PDF viewers usually does not change.
  • …and is not encoded in Unicode (e.g. “Th” in Linux Libertine):
    No change.

Testcase

(AFAIK, /ToUnicode is not included in testit.)

#set page(width: 30em, height: auto)

ffi Th ffi
#set text(ligatures: false)
ffi Th ffi

$integral_x y dif z$

#set text(lang: "zh", region: "CN", font: "Source Han Serif", fallback: false)

#let example-Han = [
  地球发动机是人类建造的力量最大的机器，比如我们所在的华北794号，全功率运行时能向大地产生150亿吨的推力。

  - ⼒ U+2F12 (Kangxi radical)
  - 力 U+529B (CJK unified)
  - 力 U+F98A (CJK compatibility)
]

#example-Han

#set text(fill: gradient.linear(..color.map.rainbow))
#example-Han

#set text(stroke: red)
#example-Han

TODO

• Remove unicode_properties. (still an indirect dependency through rustybuzz)

Future work

• Write /ActualText. (tracked by PDF text extraction can fail in complex shaping scenarios #4225 and Different characters that use the same glyph result in the same character when copied from PDFs #526)
• Use the most common instead of the first occurrence. I don't think we should use CJK unified even if the author always writes CJK compatibility.