Custom Border Using Stack

import 'package:flutter/material.dart';

void main() => runApp(new MaterialApp(home: new app()));

class app extends StatefulWidget {
  @override
  _appState createState() => new _appState();
}

class _appState extends State<app> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: Stack(
          children: <Widget>[
            ClipPath(
              clipper: CustomClip(),
                child: Image.network(
              "http://www.delonghi.com/Global/recipes/multifry/pizza_fresca.jpg",
              width: double.infinity,
              height: 400.0,
                  fit: BoxFit.cover,
            )),
            CustomPaint(
              painter: BorderPainter(),
              child: Container(
                height: 400.0,
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class BorderPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 10.0
      ..color = Colors.red;
    Path path = Path();
//    uncomment this and will get the border for all lines
    path.lineTo(0.0, 0.0);
    path.lineTo(size.width, 0.0);
    path.lineTo(size.width, size.height * 0.8);
    path.lineTo(0.0, size.height);
    path.close();
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

class CustomClip extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    Path path = Path();
    path.lineTo(0.0, 0.0);
    path.lineTo(size.width, 0.0);
    path.lineTo(size.width, size.height * 0.8);
    path.lineTo(0.0, size.height);
    path.close();
    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => true;
}

How can I add a shadow - blur - to the border color?