This commit is a result of debugging a problem with a particular font
where a VS2 variation of a glyph produced incorrect shaping.
Further investigation showed that this was specific to using freetype
font functions and that switching to opentype functions produces the
correct shaping information in that case.
Further-further investigation showed that this difference in behavior
was really due to the font file being out of spec and freetype returning
unexpected data as a result.
This commit allows switching to using harfbuzz's own opentype font+face
(rather than freetype) and/or switching the freetype-based font to using
opentype font functions. These two options don't yield equivalent
results in the wezterm integration: a couple of our shaping tests
fail due to the x-advances not being the same. Those can be drastically
different in some cases (eg: I seem to get 0 for certain bitmap strikes,
and for Roboto the value is off by a factor of about 1.5).
This is incomplete and not something I want to turn on by default at
the moment, but I don't want to lose this work, hence this commit.
refs: https://github.com/wez/wezterm/issues/2475
refs: https://github.com/harfbuzz/harfbuzz/issues/3806
