Fixing a FIXME (#1229): eliminate non-visible elements from snapping coordinates
Fixing a FIXME (#1229): eliminate non-visible elements from snapping coordinates

fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/screenshots,fzzzy/pageshot,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/screenshots