Auto merge of #7724 - glennw:expire-anims, r=pcwalton

Ensure that animations expire correctly and stop compositing occurring after they finish.

There were two problems here:

(1) The animation state update function was only called when nodes were dirty or there were new animations.
(2) When all animations for a node expired, the entry from the hash table was not removed.

The result was that once an animation began, the compositor would be running as fast as it can forever.

Fixes #7721.

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components/msg/constellation_msg.rs

@@ -275,7 +275,7 @@ pub enum Msg {
     NodeStatus(Option<String>),
 }
 
-#[derive(Clone, Eq, PartialEq, Deserialize, Serialize)]
+#[derive(Clone, Eq, PartialEq, Deserialize, Serialize, Debug)]
 pub enum AnimationState {
     AnimationsPresent,
     AnimationCallbacksPresent,