copy method

@override SmoothLine copy()

复制实例，避免对象引用传递

Copy instance to avoid object reference passing

Implementation

@override
SmoothLine copy() => SmoothLine(
      brushPrecision: brushPrecision,
      minPointDistance: minPointDistance,
      useBezierCurve: useBezierCurve,
      smoothLevel: smoothLevel,
    );