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/lib/main.dart

@@ -0,0 +1,59 @@
+import 'dart:ui' as ui;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_shaders/flutter_shaders.dart';
+
+void main() {
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Simple SDF Demo',
+      theme: ThemeData(colorSchemeSeed: Colors.blue),
+      home: const MyHomePage(),
+    );
+  }
+}
+
+class MyHomePage extends StatelessWidget {
+  const MyHomePage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Simple SDF Demo')),
+      body: ShaderBuilder(
+        assetKey: 'shaders/SDF.frag',
+        (context, shader, child) => CustomPaint(
+          size: MediaQuery.of(context).size,
+          painter: ShaderPainter(shader: shader),
+        ),
+        child: const Center(child: CircularProgressIndicator()),
+      ),
+    );
+  }
+}
+
+class ShaderPainter extends CustomPainter {
+  ShaderPainter({required this.shader});
+  ui.FragmentShader shader;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    shader.setFloat(0, size.width);
+    shader.setFloat(1, size.height);
+
+    final paint = Paint()..shader = shader;
+    canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height), paint);
+  }
+
+  @override
+  bool shouldRepaint(covariant CustomPainter oldDelegate) {
+    return false;
+  }
+}