Bug 1593970 - scale subpixel contribution based on ClearType level se…
…tting. r=jfkthame Differential Revision: https://phabricator.services.mozilla.com/D52441 [wrupdater] From https://hg.mozilla.org/mozilla-central/rev/d967ac6c56aeec99c2fba3814c8550a1c5971384
