switch to less stuttery timing method

Scripts/Scenes/Play/Audio/BGM.cs

@@ -4,6 +4,19 @@ using System;
 
 public partial class BGM : AudioStreamPlayer
 {
+	// latency compensation
+	private ulong timeStartUsec;
+	private float timeDelay;
+	
+	public float CurTime
+	{
+		get
+		{
+			float time = (Time.GetTicksUsec() - timeStartUsec) / 1000000f;
+			return time - timeDelay;
+		}
+	}
+
 	public void LoadFromUser(string path)
 	{
 		if (!path.StartsWith("user://"))
@@ -43,4 +56,11 @@ public partial class BGM : AudioStreamPlayer
 				break;
 		}
 	}
+
+	public void Play()
+	{
+		timeStartUsec = Time.GetTicksUsec();
+		timeDelay = (float) (AudioServer.GetTimeToNextMix() + AudioServer.GetOutputLatency());
+		base.Play();
+	}
 }

Scripts/Scenes/Play/Play.cs

@@ -124,7 +124,7 @@ namespace WacK.Scenes
 			double time = bgmController.GetPlaybackPosition() + AudioServer.GetTimeSinceLastMix() - AudioServer.GetOutputLatency();
 			
 			var nPos = noteDisplay.Position;
-			nPos.Y = ((float)time * scrollPxPerSec) + 1920;
+			nPos.Y = (bgmController.CurTime * scrollPxPerSec) + 1910;
 			noteDisplay.Position = nPos;
 			scrollDisplay.Position = nPos;
         }