Bug 1635406 made it so that webrender snaps the offsets of animated
transforms before accumulating them in to the reference frame
transform. Unfortunately, however, this causes jittery animations. The
original intention was just to snap the visual viewport offset when
scrolling, to avoid excessive picture cache invalidation.

To avoid this, make it so that we only snap for reference frames of
kind ReferenceFrameKind::Zoom. This will mean that most animations are
unaffected. There may however still be some jitter when zooming, but
this is outweighed by the benefit of not invalidating picture cache
tiles every frame when scrolling.

Differential Revision: https://phabricator.services.mozilla.com/D77679