style: Send offset-position to the compositor

We send offset-position to the compositor, just like other similar
properties, e.g. offset-rotate, offset-anchor. This includes extracting
this animation value, doing serialization and sending it via IPC.

So now we can run the animation of ray() on the compositor properly.

Differential Revision: https://phabricator.services.mozilla.com/D179862

components/style/properties/longhands/box.mako.rs

@@ -273,6 +273,7 @@ ${helpers.predefined_type(
     engines="gecko",
     animation_value_type="ComputedValue",
     gecko_pref="layout.css.motion-path-offset-position.enabled",
+    flags="CAN_ANIMATE_ON_COMPOSITOR",
     spec="https://drafts.fxtf.org/motion-1/#offset-position-property",
     servo_restyle_damage="reflow_out_of_flow",
     boxed=True