add Dart/headless example

examples/dart/cli_headless/.gitignore

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+# https://dart.dev/guides/libraries/private-files
+# Created by `dart pub`
+.dart_tool/
+output/**

examples/dart/cli_headless/CHANGELOG.md

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+## 1.0.0
+
+- Initial version.

examples/dart/cli_headless/README.md

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+# cli_headless
+
+An example of Thermion running via Dart CLI with no hardware accelerated rendering surface.

examples/dart/cli_headless/analysis_options.yaml

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+# This file configures the static analysis results for your project (errors,
+# warnings, and lints).
+#
+# This enables the 'recommended' set of lints from `package:lints`.
+# This set helps identify many issues that may lead to problems when running
+# or consuming Dart code, and enforces writing Dart using a single, idiomatic
+# style and format.
+#
+# If you want a smaller set of lints you can change this to specify
+# 'package:lints/core.yaml'. These are just the most critical lints
+# (the recommended set includes the core lints).
+# The core lints are also what is used by pub.dev for scoring packages.
+
+include: package:lints/recommended.yaml
+
+# Uncomment the following section to specify additional rules.
+
+# linter:
+#   rules:
+#     - camel_case_types
+
+# analyzer:
+#   exclude:
+#     - path/to/excluded/files/**
+
+# For more information about the core and recommended set of lints, see
+# https://dart.dev/go/core-lints
+
+# For additional information about configuring this file, see
+# https://dart.dev/guides/language/analysis-options

examples/dart/cli_headless/bin/example.dart

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+import 'dart:io';
+import 'dart:isolate';
+
+import 'package:thermion_dart/thermion_dart.dart';
+import 'package:thermion_dart/src/filament/src/implementation/ffi_filament_app.dart';
+
+void main() async {
+  await FFIFilamentApp.create();
+  final (width, height) = (500, 500);
+  final sc = await FilamentApp.instance!.createHeadlessSwapChain(width, height);
+  var viewer = ThermionViewerFFI();
+  await viewer.initialized;
+
+  await FilamentApp.instance!.register(sc, viewer.view);
+
+  await viewer.view.setFrustumCullingEnabled(false);
+  await viewer.setBackgroundColor(1, 0, 1, 1);
+  await viewer.setViewport(width, height);
+  final result = await FilamentApp.instance!.capture(
+    sc,
+    view: viewer.view,
+  );
+
+  final bitmap = await pixelBufferToBmp(result.first.$2, width, height,
+      hasAlpha: true, isFloat: true);
+
+  var outfile = File("output/render.bmp");
+  outfile.parent.create();
+  outfile.writeAsBytesSync(bitmap);
+  await FilamentApp.instance!.destroy();
+  Isolate.current.kill();
+}

examples/dart/cli_headless/pubspec.yaml

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+name: example_cli
+description: A sample command-line application.
+version: 1.0.0
+
+environment:
+  sdk: ^3.3.0
+
+dependencies:
+  thermion_dart: 
+    path: ../../../thermion_dart
+
+dev_dependencies:
+  ffigen: ^11.0.0
+  lints: ^3.0.0
+  test: ^1.24.0