animation: fix crash with zero-duration animations

src/animation.rs

@@ -115,7 +115,7 @@ struct Animator<'a, T, const ACTOR_COUNT: usize> {
 impl<'a, T, const ACTOR_COUNT: usize> Animator<'a, T, ACTOR_COUNT> {
 }
 
-impl<T> Animation<T> {
+impl<T: core::fmt::Debug> Animation<T> {
     pub const fn new() -> Self {
         Self {
             from: None,
@@ -128,12 +128,14 @@ impl<T> Animation<T> {
         let mut now: Instant = Instant::now();
         loop {
             // Find the next shortest delay
-            let mut next_keyframe_time = animators[0].next_update;
+            let mut next_keyframe_time = Instant::MAX;
             let mut finished = false;
+            let mut has_valid = false;
             for animator in &mut animators {
                 if !animator.is_valid() {
                     continue;
                 }
+                has_valid = true;
                 if animator.next_update <= now {
                     finished = match animator.tick() {
                         TickResult::Finished => true,
@@ -146,10 +148,13 @@ impl<T> Animation<T> {
                 }
             }
 
+            // If there are no valid animators, or if all animators are finished, then we're done
+            finished |= !has_valid;
             if finished {
                 break;
             }
 
+            assert!(next_keyframe_time > now, "Weird times: {next_keyframe_time:?} is earlier than {now:?} animators={animators:?}");
             let keyframe_delay = next_keyframe_time - now;
             trace!("delay {:?}", keyframe_delay.as_millis());
             Timer::after(keyframe_delay).await;