Stop transitioning properties with a discrete animation type (#30565)

They are not transitionable:
https://drafts.csswg.org/css-transitions-1/#transitionable

There are some new failures in background-image-interpolation.html,
but I think the test is wrong, because it expects background-image
to be transitionable, even though the spec defines it with a discrete
animation type.

components/style/animation.rs

@@ -1028,6 +1028,10 @@ impl ElementAnimationSet {
         old_style: &ComputedValues,
         new_style: &Arc<ComputedValues>,
     ) {
+        if !longhand_id.is_transitionable() {
+            return;
+        }
+
         let style = new_style.get_ui();
         let timing_function = style.transition_timing_function_mod(index);
         let duration = style.transition_duration_mod(index);