Blend the animation playback track when timestamp value is manually changed

[Miguel0312]

• This closes AnimationPlayerEditor doesn't preview the Animation when seeking Animation Playback Tracks #86217

Before, the tracks of type Animation Playback weren't recalculated when the timeline was changed manually, it was only updated when the track's animation was played. Now, we can manually set a time value and see how the animation will look at that instant, allowing to more easily sync multiple animations.

Preview

I created a simple AnimationPlayer that makes a blinking sprite and connected it as a track in another AnimationPlayer.

[TokageItLab]

The AnimationPlaybackTrack key triggers playback, not updates. It means that another AnimationPlayer that is assigned may have AudioTracks, MethodTracks, etc., and they must not be fired when update_only.

At least the third argument of player2->seek() should propagate p_update_only. But I still wonder if that is enough to guarantee safety.

By the way, with the AnimationMixer implementation, AnimationPlayer can theoretically be externally blended with arbitrary weights. My plan is to implement an api that allows AnimationPlaybackTrack to control the animation externally with more flexibility than play() and seek() as the similar way with AudioPlaybackTrack, as I mentioned in #70338 (comment).

[TokageItLab]

I sent a proposal for correct implementation to handle AnimationPlaybackTrack godotengine/godot-proposals#8783

[akien-mga]

Superseded by #89794. Thanks for the contribution!