code: private Path getOuterBorder(Point[] points) {
          Path path = new Path();
          path.moveTo(points[0].x, points[0].y);
          for (int i = 1; i < _corners; i++) {
              path.lineTo(points[i].x, points[i].y);
          }
          path.close();
          return path;
      }
docs: 获取外边框