[skrifa] Glyphs stem-darkening configuration

[maximilionus]

The FreeType library allows the detailed top-level rendering configurations through the environmental variable. That's very handy to configure the stem-darkening parameters on Linux systems, where it is disabled by default almost in every distribution, as you can simply enable it by just passing a few values through the FREETYPE_PROPERTIES variable.

To enable the basic stem-darkening it was enough to set the no-stem-darkening variable for the selected driver:

$ FREETYPE_PROPERTIES="autofitter:no-stem-darkening=0" chromium

Will this project support something like that, at least on some bare minimal level of just on/off switches? Because font rendering without darkening is just completely unbearable on Linux for me.

[maximilionus]

Sadly, I see that stem-darkening is not even supported yet by this project as the #1407 says.