feat: add the ability to use Sointu as a sync-tracker

There is a new "sync" opcode that saves the top-most signal every 256 samples to the new "syncBuffer" output. Additionally, you can enable saving the current fractional row as sync[0], avoiding calculating the beat in the shader, but also calculating the beat correctly when the beat is modulated.

cmd/sointu-play/main.go

@@ -92,7 +92,7 @@ func main() {
 				synth.Release(i)
 			}
 		}
-		buffer, err := sointu.Play(synth, song) // render the song to calculate its length
+		buffer, _, err := sointu.Play(synth, song) // render the song to calculate its length
 		if err != nil {
 			return fmt.Errorf("sointu.Play failed: %v", err)
 		}