Build tools for Flutter GPU shader bundles/libraries.

Use native asset build hooks to import Flutter GPU shader bundle assets.

1. This package requires the experimental "native assets" feature to be enabled. Enable it with the following command:

   flutter config --enable-native-assets

2. Place some Flutter GPU shaders in your project. For this example, we'll assume the existence of two shaders: shaders/my_cool_shader.vert and shaders/my_cool_shader.frag.
3. Create a shader bundle manifest file in your project. The filename must end with .shaderbundle.json. For this example, we'll assume the following file is saved as my_cool_bundle.shaderbundle.json:

   {
       "CoolVertex": {
           "type": "vertex",
           "file": "shaders/my_cool_shader.vert"
       },
       "CoolFragment": {
           "type": "fragment",
           "file": "shaders/my_cool_shader.frag"
       }
   }

4. Next, define a build hook in your project that builds the shader bundle using buildShaderBundleJson. The build hook must be named hook/build.dart in your project; this script will be automatically invoked by Flutter when the "native assets" feature is enabled:

   import 'package:native_assets_cli/native_assets_cli.dart';
   import 'package:flutter_gpu_shaders/build.dart';
   
   void main(List<String> args) async {
     await build(args, (config, output) async {
       await buildShaderBundleJson(
           buildConfig: config,
           buildOutput: output,
           manifestFileName: 'my_cool_bundle.shaderbundle.json');
     });
   }

5. In your project's pubspec.yaml, add an asset import rule to package the built shader bundles (this will become unnecessary once "native assets" supports DataAsset in a future release of Flutter):

   flutter:
     assets:
       - build/shaderbundles/*.shaderbundle.json

6. You can now import the built shader bundle as a library using gpu.ShaderLibrary.fromAsset in your project. For example:

   import 'package:flutter_gpu/gpu.dart' as gpu;
   
   final String _kBaseShaderBundlePath =
       'packages/my_project/build/shaderbundles/my_cool_bundle.shaderbundle';
   
   gpu.ShaderLibrary? _baseShaderLibrary = null;
   gpu.ShaderLibrary get baseShaderLibrary {
     if (_baseShaderLibrary != null) {
       return _baseShaderLibrary!;
     }
     _baseShaderLibrary = gpu.ShaderLibrary.fromAsset(_kBaseShaderBundlePath);
     if (_baseShaderLibrary != null) {
       return _baseShaderLibrary!;
     }
   
     throw Exception(
         "Failed to load base shader bundle! ($_kBaseShaderBundlePath)");
   }