libservo: Move animation tracking from WindowMethods to delegates (#36400)

This changes removes animation tracking from the `WindowMethods` trait
and moves it to `ServoDelegate` and `WebViewDelegate`.

- Animation changes per-`WebView` are now triggered in the compositor
  only when the value is updated there, rather than right after ticking
  animations.
- Both `WebView` and `Servo` now expose an `animation()` method, so
  tracking animation state actually becomes unecessary in many cases,
  such as that of desktop servoshell, which can just read the value
  when the event loop spins.

Testing: No tests necessary as the API layer is still untested. Later,
tests will be added for the `WebView` API and this can be tested then.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/compositing/lib.rs

@@ -590,4 +590,5 @@ impl From<SerializableImageData> for ImageData {
 pub trait RendererWebView {
     fn id(&self) -> WebViewId;
     fn screen_geometry(&self) -> Option<ScreenGeometry>;
+    fn set_animating(&self, new_value: bool);
 }