[ft-devel] Stem darkening toggling API

[Nikolaus Waxweiler]

Hey list,
so I finally got around implementing a concept: a new API entry point 
`FT_Face_SetStemDarkening( FT_Face face, FT_Bool enable )`. It toggles a 
new member of FT_FaceRec `no_stem_darkening`. Font drivers and the 
autohinter poll this member to decide if they must darken stems. 
`FT_Load_Glyph(...)` now modifies the load flags because only the CFF 
driver darkens stems both when and when not hinting, all other cases 
need the light autohinter. This means you're currently limited to 
autohinting on all drivers except the CFF one. It currently somewhat 
breaks `FT_LOAD_TARGET_LCD` though, must see about that still. And I may 
have gotten some combinatorial flag cases wrong.

https://github.com/madig/freetype2/tree/stem-darkening-api

To make this work more orthogonally, I'd need to implement stem 
darkening in the TT and T1 drivers when not hinting and when hinting in 
v40 compatibility mode (at least horizontally). Other TT hinting modes 
are conceptually incompatible with stem darkening due to the explicit 
nature of TT programming.

To get rid of duplication, the `no_stem_darkening` property should 
probably be removed from the drivers and the darkening_parameters moved 
to FT_Face.

So, comments?