…y. r=lsalzman

The glyph pixel space in which we rasterized glyphs differed from how we
rendered the rasterized glyphs in the shader. They need to be in
agreement because the glyph subpixel offset selected during
rasterization depends on it. This patch should make the paths consistent
with each other.

Additionally, during animations, we now snap the reference frame
relative offset ignoring the impact of any animated transforms. This
helps with minimizing glyph wiggling during the transition.

Differential Revision: https://phabricator.services.mozilla.com/D51305

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/1e12f9b1812772fee75f80a38fd27b5fff8cffbc