[Development] Font rendering on X11/Wayland: New FreeType API to toggle LCD filters and stem darkening on a per-font basis, would love a review

[Nikolaus Waxweiler]

>> If the new FT_Face_Option() is present, FT can do stem darkening on
>> (almost entirely) all outline fonts :) The toolkit just has to invoke
>> the function on all FT_Faces to be rendered.
> 
> But will stem darkening work with the bytecode interpreter?

It won't until I implement it in the TrueType engine, and only for the
v40 interpreter in backwards compatibility mode. I don't think the
concept of stem darkening is compatible with the explicit glyph
modifications on both axes that are possible in v40 native ClearType
mode and v35.

Turning on stem darkening currently triggers the autohinter for all
outlines formats except .otf, where the font driver can do it natively.