Stop Timeline playableAsset overlap each other.

如果不希望這種情況, 需要使用 UnityEngine.Timelines.ITimelineClipAsset, 中的 ClipCaps 來標示

using UnityEngine.Playables;
using UnityEngine.Timeline;

public abstract class PlayableAssetWithoutBlend : PlayableAsset, ITimelineClipAsset
{
	public ClipCaps clipCaps => ClipCaps.None;
}

implement 後, 在 timeline 中相關的 playableAsset 就不能再 overrlap blending.
解決 script 沒有 blend state 的問題.