This will be used in order to hold animations for pseudo elements in the
DocumentAnimationSet. Also no longer store the OpaqueNode in the
animation and transition data structures. This is already part of the
DocumentAnimationSet key.
Instead of having `ScriptThread` handle rooting nodes, do this in
`Animations`. This makes it easier to know when it is appropriate to
root and unroot nodes instead of relying on a certain order of events.
This also allows reducing quite a bit the amount of unsafe code.
For legacy reasons, transitions were marked as finished after updating the
style. According to the spec, they should be marked as finished when
animations are updated and before restyle. This change does that as well
as preventing replacement of finished transitions.
Having finished transitions survive a full restyle cycle and allowing
the replacement of finished transition, could lead to issues were
removed from the global list of animating transitions too soon:
1. A transitions finishes
2. Restyle
3. Transitions is marked as finished and events are queued
4. Restyle cancels finished transition and replaces it instead of
clearing finished transition
5. Events are sent for the incorrectly canceled transition removing it
completely from the list of running transitions due to the extra
event.
This event is triggered when an animation iterates. This change also
moves iteration out of style calculation to an "update animations" which
is the next part of having animation event handling match the HTML spec.
This removes an extra layer of abstraction and allows Servo to share
more code with Gecko. In addition, we will need to handle raw
`AnimationValue` structs soon in order to fully implement "faster
reversing of interrupted transitions."
This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.
We still look for nodes not in the flow tree in the layout thread.
