fix: Make build_shaders script compatible with other platforms

packages/flame_3d/bin/build_shaders.dart

@@ -57,7 +57,8 @@ Future<void> compute(Directory assets, Directory shaders) async {
     };
 
     stdout.writeln('Computing shader "$name"');
-    final result = await Process.run(impellerC, [
+    final impellerC = await findImpellerC();
+    final result = await Process.run(impellerC.toFilePath(), [
       '--sl=${assets.path}/$name.shaderbundle',
       '--shader-bundle=${jsonEncode(bundle)}',
     ]);
@@ -68,5 +69,97 @@ Future<void> compute(Directory assets, Directory shaders) async {
   }
 }
 
-final impellerC =
-    '${Platform.environment['FLUTTER_HOME']}/bin/cache/artifacts/engine/darwin-x64/impellerc';
+// Copied from https://github.com/bdero/flutter_gpu_shaders/blob/master/lib/environment.dart#L53
+const _macosHostArtifacts = 'darwin-x64';
+const _linuxHostArtifacts = 'linux-x64';
+const _windowsHostArtifacts = 'windows-x64';
+
+const _impellercLocations = [
+  '$_macosHostArtifacts/impellerc',
+  '$_linuxHostArtifacts/impellerc',
+  '$_windowsHostArtifacts/impellerc.exe',
+];
+
+/// Locate the engine artifacts cache directory in the Flutter SDK.
+Uri findEngineArtifactsDir({String? dartPath}) {
+  // Could be:
+  //   `/path/to/flutter/bin/cache/dart-sdk/bin/dart`
+  //   `/path/to/flutter/bin/cache/artifacts/engine/darwin-x64/flutter_tester`
+  //   `/path/to/.puro/shared/caches/94cf8c8fad31206e440611e309757a5a9b3be712/dart-sdk/bin/dart`
+  final dartExec = Uri.file(dartPath ?? Platform.resolvedExecutable);
+
+  Uri? cacheDir;
+  // Search backwards through the segment list until finding `bin` and `cache` in sequence.
+  for (var i = dartExec.pathSegments.length - 1; i >= 0; i--) {
+    if (dartExec.pathSegments[i] == 'dart-sdk' ||
+        dartExec.pathSegments[i] == 'artifacts') {
+      // Note: The final empty string denotes that this is a directory path.
+      cacheDir = dartExec.replace(
+        pathSegments: dartExec.pathSegments.sublist(0, i) + [''],
+      );
+      break;
+    }
+  }
+  if (cacheDir == null) {
+    throw Exception(
+      'Unable to find Flutter SDK cache directory! '
+      'Dart executable: `${dartExec.toFilePath()}`',
+    );
+  }
+  // We should now have a path of `/path/to/flutter/bin/cache/`.
+
+  final engineArtifactsDir = cacheDir
+      .resolve('./artifacts/engine/'); // Note: The final slash is important.
+
+  return engineArtifactsDir;
+}
+
+/// Locate the ImpellerC offline shader compiler in the engine artifacts cache
+/// directory.
+Future<Uri> findImpellerC() async {
+  /////////////////////////////////////////////////////////////////////////////
+  /// 1. If the `IMPELLERC` environment variable is set, use it.
+  ///
+
+  // ignore: do_not_use_environment
+  const impellercEnvVar = String.fromEnvironment('IMPELLERC');
+  if (impellercEnvVar != '') {
+    if (!doesFileExist(impellercEnvVar)) {
+      throw Exception(
+        'IMPELLERC environment variable is set, '
+        "but it doesn't point to a valid file!",
+      );
+    }
+    return Uri.file(impellercEnvVar);
+  }
+
+  /////////////////////////////////////////////////////////////////////////////
+  /// 3. Search for the `impellerc` binary within the host-specific artifacts.
+  ///
+
+  final engineArtifactsDir = findEngineArtifactsDir();
+
+  // No need to get fancy. Just search all the possible directories rather than
+  // picking the correct one for the specific host type.
+  Uri? found;
+  final tried = <Uri>[];
+  for (final variant in _impellercLocations) {
+    final impellercPath = engineArtifactsDir.resolve(variant);
+    if (doesFileExist(impellercPath.toFilePath())) {
+      found = impellercPath;
+      break;
+    }
+    tried.add(impellercPath);
+  }
+  if (found == null) {
+    throw Exception(
+      'Unable to find impellerc! Tried the following locations: $tried',
+    );
+  }
+
+  return found;
+}
+
+bool doesFileExist(String path) {
+  return File(path).existsSync();
+}