…ansforms. r=nical

We incorporate the reference frame origin offset for non-animated
transforms, but forgot for animated transforms. Since the offset itself
is not animated, we should still incorporate it into the snapping
transform.

Differential Revision: https://phabricator.services.mozilla.com/D61689

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/a2ed78dab7bcf0cf5a0ff49606cafdd55b2f2fd9