v0.5.0: fixed many vfx alignment issues

SPD-classes/src/main/java/com/watabou/noosa/Visual.java

@@ -147,6 +147,14 @@ public class Visual extends Gizmo {
 		y = p.y - height / 2;
 		return p;
 	}
+
+	//returns the point needed to center the argument visual on this visual
+	public PointF center( Visual v ) {
+		return new PointF(
+				x + (width() - v.width())/2f,
+				y + (height() - v.height())/2f
+		);
+	}
 	
 	public float width() {
 		return width * scale.x;