Add new sample simple_sdf (#2835)

Add a new sample, `simple_sdf`. This is similar to `simple_shader`, but
it focuses on using Signed Distance Functions within the fragment
shader.

Relates to issue
[#183043](https://github.com/flutter/flutter/issues/183043)

## 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].
- [ ] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


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

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

simple_sdf/shaders/SDF.frag

@@ -0,0 +1,62 @@
+#version 460 core
+
+#include <flutter/runtime_effect.glsl>
+
+precision mediump float;
+
+uniform vec2 resolution;
+out vec4 fragColor;
+
+vec3 pink = vec3(255, 105, 180) / 255;
+
+// dot2 and sdHeart from https://iquilezles.org/articles/distfunctions2d/
+//
+// The MIT License
+// Copyright © 2015 Inigo Quilez
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions: The above copyright
+// notice and this permission notice shall be included in all copies or
+// substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS",
+// WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR
+// THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+// https://www.youtube.com/c/InigoQuilez
+// https://iquilezles.org
+
+float dot2(vec2 v) { return dot(v, v); }
+float sdHeart(in vec2 p) {
+  p.x = abs(p.x);
+
+  if (p.y + p.x > 1.0)
+    return sqrt(dot2(p - vec2(0.25, 0.75))) - sqrt(2.0) / 4.0;
+  return sqrt(min(dot2(p - vec2(0.00, 1.00)),
+                  dot2(p - 0.5 * max(p.x + p.y, 0.0)))) *
+         sign(p.x - p.y);
+}
+
+void main() {
+  vec2 st = FlutterFragCoord().xy / resolution.xy;
+  // Remap coordinates.
+  // Flutter normalized coordinates have range [0,1] but sdHeart expects [-1,1].
+  st = (st - vec2(0.5)) * vec2(2.0);
+  // Center the heart.
+  // sdHeart is written such that the bottom point is at (0,0) and it's about 1
+  // unit tall.
+  st.y -= 0.5;
+  // Invert Y coordinate.
+  st.y *= -1;
+
+  // Calculate the color of this pixel according to the heart SDF, using
+  // smoothstep to anti-alias the edges.
+  vec3 color = vec3(0.0);
+  color = mix(pink, color, smoothstep(0.01, 0.02, sdHeart(st)));
+
+  fragColor = vec4(color, 1);
+}