add Dart/headless example

examples/dart/cli_headless/bin/example.dart

@@ -0,0 +1,32 @@
+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();
+}