Bug 1587094 - Create a pref to control whether we use DirectWrite's bold simulation or multi-strike synthetic bold; default to multi-strike for webfonts. r=lsalzman
Bug 1587094 - Create a pref to control whether we use DirectWrite's bold simulation or multi-strike synthetic bold; default to multi-strike for webfonts. r=lsalzman

This is designed to mitigate the problem of third-party fonts that render poorly
under DirectWrite's bold simulation, by using multi-strike synthetic bold (like
we use on macOS) instead.

The behavior is controlled by a pref, so that we can readily switch between
using DWrite's bold simulation for all fonts (pref=2, our current behavior);
using it only for installed fonts and falling back to multi-strike for webfonts
(pref=1, new behavior); or never using the DWrite simulation (pref=0).

Differential Revision: https://phabricator.services.mozilla.com/D137584

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/c885ece8496139420bfdc9ab9305f2a7e7ff21e5

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender