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/assets/shaders/common/common.glsl

@@ -0,0 +1,16 @@
+// 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.
+
+#define M_PI 3.14159265
+#define M_INVPI 0.31830989
+
+float hash_1d(float v) {
+  float u = 50.0 * sin(v * 3000.0);
+  return 2.0 * fract(2.0 * u * u) - 1.0;
+}
+
+float hash_2d(vec2 pos) {
+  vec2 uv = 50.0 * fract(pos * M_INVPI);
+  return 2.0 * fract(uv.x * uv.y * (uv.x + uv.y)) - 1.0;
+}