Better computation of animation keyframes

This begins to address #26625 by properly applying CSS variables during
keyframe computation and no longer using `apply_declarations`. Instead,
walk the declarations, combining them into IntermediateComputedKeyframe,
maintaining declarations that modify CSS custom properties. Then compute
a set of AnimationValues for each keyframe and use those to produce
interpolated animation values.

components/layout_thread/dom_wrapper.rs

@@ -341,6 +341,10 @@ impl<'ld> TDocument for ServoLayoutDocument<'ld> {
     fn is_html_document(&self) -> bool {
         self.document.is_html_document_for_layout()
     }
+
+    fn shared_lock(&self) -> &StyleSharedRwLock {
+        self.document.style_shared_lock()
+    }
 }
 
 impl<'ld> ServoLayoutDocument<'ld> {