Currently we try to alternate between light-on-dark and dark-on-light
text rendering masks on macOS depending on text colors. Historically,
this matched earlier versions of macOS reasonably well. In successive
versions of macOS, however, this approximation appears to have broken
down and no longer gives correct results.
Given that recent macOS versions no longer promotes font-smoothing at
all, it seems more reasonable to match Skia's font smoothing handling
because it appears to better work with these more recent versions while
requiring us to have less special code to do so.
Differential Revision: https://phabricator.services.mozilla.com/D229444
