Fix repeated animation transition bug (#14411)

# Objective

Fixes #13910

When a transition is over, the animation is stopped. There was a race
condition; if an animation was started while it also had an active
transition, the transition ending would then incorrectly stop the newly
added animation.

## Solution

When starting an animation, cancel any previous transition for the same
animation.

## Testing

The changes were tested manually, mainly by using the `animated_fox`
example. I also tested with changes from
https://github.com/bevyengine/bevy/pull/13909.

I'd like to have an unit test for this as well, but it seems quite
complex to do, as I'm not sure how I would detect an incorrectly paused
animation.

Reviewers can follow the instructions in #13910 to reproduce.

Tested on macos 14.4 (M3 processor) Should be platform-independent,
though.

crates/bevy_animation/src/transition.rs

@@ -92,7 +92,11 @@ impl AnimationTransitions {
             }
         }
 
-        self.main_animation = Some(new_animation);
+        // If already transitioning away from this animation, cancel the transition.
+        // Otherwise the transition ending would incorrectly stop the new animation.
+        self.transitions
+            .retain(|transition| transition.animation != new_animation);
+
         player.start(new_animation)
     }