Next Gen UI demo (#1778)

First pass at a Next Generation UI demo app. The UI needs work, feedback
gratefully accepted.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/lib/common/shader_painter.dart

@@ -0,0 +1,28 @@
+// Copyright 2023 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui';
+
+import 'package:flutter/material.dart';
+
+class ShaderPainter extends CustomPainter {
+  ShaderPainter(this.shader, {this.update});
+
+  final FragmentShader shader;
+  final void Function(FragmentShader, Size)? update;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    update?.call(shader, size);
+    canvas.drawRect(
+      Rect.fromLTWH(0, 0, size.width, size.height),
+      Paint()..shader = shader,
+    );
+  }
+
+  @override
+  bool shouldRepaint(covariant ShaderPainter oldDelegate) {
+    return oldDelegate.shader != shader || oldDelegate.update != update;
+  }
+}