Bug 1613375 - Always use reference frame origin offset in snapping tr…

…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

webrender/src/spatial_node.rs

@@ -699,8 +699,12 @@ impl SpatialNode {
                     }
 
                     // Assume animations start at the identity transform for snapping purposes.
+                    // We still want to incorporate the reference frame offset however.
                     // TODO(aosmond): Is there a better known starting point?
-                    PropertyBinding::Binding(..) => ScaleOffset::identity(),
+                    PropertyBinding::Binding(..) => {
+                        let origin_offset = info.origin_in_parent_reference_frame;
+                        ScaleOffset::from_offset(origin_offset.to_untyped())
+                    }
                 }
             }
             _ => ScaleOffset::identity(),