… offsets r=gw

Since the filtered-backdrop is 1:1 with the backdrop, we don't want any snap offsets to shift the contents

Differential Revision: https://phabricator.services.mozilla.com/D42042

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/9e513ae4747fad12001ef627e6f273624a3feff4