Implement hit object stacking algorithm.

src/itdelatrisu/opsu/objects/Spinner.java

@@ -263,6 +263,12 @@ public class Spinner implements HitObject {
 		return false;
 	}
 
+	@Override
+	public OsuHitObject getHitObject() { return hitObject; }
+
+	@Override
+	public void updatePosition() {}
+
 	@Override
 	public float[] getPointAt(int trackPosition) {
 		// get spinner time