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/ui_glitch.frag

@@ -0,0 +1,68 @@
+// 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.
+
+#version 460 core
+
+#include "common/common.glsl"
+#include <flutter/runtime_effect.glsl>
+
+uniform vec2 uResolution;
+uniform float uTime;
+uniform sampler2D uTex;
+
+out vec4 oColor;
+
+float cubicPulse(float c, float w, float x) {
+	x = abs(x - c);
+	if (x > w)
+		return 0.0;
+	x /= w;
+	return 1.0 - x*x*(3.0 - 2.0*x);
+}
+
+float twoSin(float x) {
+	x = 6.49*x - 0.65;
+	float t = -0.7*sin(6.8*x) + 1.4*sin(2.9*x);
+	t = t/4.1 + 0.5;
+	return t;
+}
+
+
+void main() {
+
+  vec2 uv = vec2(FlutterFragCoord().xy) / uResolution;
+
+  float t_2 = cubicPulse(.5, .05, fract(uTime / 4.0));
+  float t_1 = twoSin(fract(uTime / 5.0));
+  float glitchScale = mix(0.0, 8.0, t_1 + t_2);
+  float aberrationSize = mix(0.0, 5.0, t_1 + t_2);
+
+  float h = hash_1d(uv.y);
+  float hs = sign(h);
+  h = max(h, 0.0);
+  h = h * h;
+  h = round(h) * hs;
+
+  uv += vec2(h * glitchScale, 0.0) / uResolution;
+
+  vec2 redOffset = vec2(aberrationSize, 0.0) / uResolution;
+  vec2 greenOffset = vec2(0.0, 0.0) / uResolution;
+  vec2 blueOffset = vec2(-aberrationSize, 0.0) / uResolution;
+
+  vec2 redUv = uv + redOffset;
+  vec2 greenUv = uv + greenOffset;
+  vec2 blueUv = uv + blueOffset;
+
+  vec2 ra = texture(uTex, redUv).ra;
+  vec2 ga = texture(uTex, greenUv).ga;
+  vec2 ba = texture(uTex, blueUv).ba;
+
+  // Convert from pre-multiplied alpha
+  ra.x /= ra.y;
+  ga.x /= ga.y;
+  ba.x /= ba.y;
+
+  float alpha = max(ra.y, max(ga.y, ba.y));
+  oColor = vec4(ra.x, ga.x, ba.x, 1.0) * alpha;
+}