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/style/properties/declaration_block.rs

@@ -321,6 +321,13 @@ impl PropertyDeclarationBlock {
         self.longhands.contains_any_reset()
     }
 
+    /// Returns a `LonghandIdSet` representing the properties that are changed in
+    /// this block.
+    #[inline]
+    pub fn longhands(&self) -> &LonghandIdSet {
+        &self.longhands
+    }
+
     /// Get a declaration for a given property.
     ///
     /// NOTE: This is linear time in the case of custom properties or in the