fix(cursor): delegate native image scaling to cursor plugin

Use the original Flutter image and source hotspot with explicit DPR, retaining the existing view and minimum-size policies. Pin the plugin implementation from rustdesk-org/flutter_custom_cursor#1.

flutter/test/cursor_plugin_test.dart

@@ -0,0 +1,108 @@
+// These tests drive the lifecycle normally owned by MouseTracker.
+// ignore_for_file: invalid_use_of_protected_member
+
+import 'dart:io';
+import 'dart:ui' as ui;
+
+import 'package:flutter/services.dart';
+import 'package:flutter_hbb/models/model.dart';
+import 'package:flutter_hbb/native/custom_cursor.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+
+class _CursorModel implements CursorModel {
+  @override
+  final Set<String> cachedKeys = {};
+  @override
+  void addKey(String key) => cachedKeys.add(key);
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+Future<CursorData> _data() async {
+  final recorder = ui.PictureRecorder();
+  ui.Canvas(recorder).drawColor(const ui.Color(0xff123456), ui.BlendMode.src);
+  final picture = recorder.endRecording();
+  final nativeImage = await picture.toImage(20, 30);
+  picture.dispose();
+  addTearDown(nativeImage.dispose);
+  return CursorData(
+      peerId: 'plugin',
+      id: 'edit',
+      image: img.Image(width: 20, height: 30, numChannels: 4),
+      nativeImage: nativeImage,
+      scale: 1,
+      data: Uint8List.fromList([1, 2]),
+      hotxOrigin: 4,
+      hotyOrigin: 16,
+      width: 20,
+      height: 30);
+}
+
+void main() {
+  final binding = TestWidgetsFlutterBinding.ensureInitialized();
+  final view = binding.platformDispatcher.views.single;
+  final channel = Platform.isWindows
+      ? SystemChannels.mouseCursor
+      : const MethodChannel('flutter_custom_cursor');
+  late List<Map<dynamic, dynamic>> registrations;
+  setUp(() {
+    registrations = [];
+    binding.defaultBinaryMessenger.setMockMethodCallHandler(channel,
+        (call) async {
+      if (call.method.startsWith('createCustomCursor')) {
+        final args = call.arguments as Map<dynamic, dynamic>;
+        registrations.add(args);
+        return args['name'];
+      }
+      return null;
+    });
+  });
+  tearDown(() {
+    view.resetDevicePixelRatio();
+    binding.defaultBinaryMessenger.setMockMethodCallHandler(channel, null);
+  });
+
+  test('native bridge delegates rasterization and retains the minimum policy',
+      () => _checkRasterization(view, registrations));
+  test('live DPR changes invalidate a cached native cursor',
+      () => _checkDprChange(view, registrations));
+}
+
+Future<void> _checkRasterization(
+    TestFlutterView view, List<Map<dynamic, dynamic>> registrations) async {
+  view.devicePixelRatio = 2;
+  final data = await _data();
+  final originalBytes = data.data;
+  final cursor = _CursorModel();
+  final session = buildCursorOfCache(cursor, 0.5, data).createSession(1);
+  await session.activate();
+  session.dispose();
+  final args = registrations.single;
+  final pixelsPerUnit = Platform.isWindows ? 1 : 2;
+  expect(data.scale, 0.6);
+  expect(identical(data.data, originalBytes), isTrue);
+  expect(args['height'], 18 * pixelsPerUnit);
+  expect(args['width'], (Platform.isLinux ? 18 : 12) * pixelsPerUnit);
+  expect(args['imagePixelRatio'], 2.0);
+  expect(args['hotX'], Platform.isMacOS ? 4.8 : 2 * pixelsPerUnit);
+  expect(args['hotY'], Platform.isMacOS ? 19.2 : 10 * pixelsPerUnit);
+  await deleteCustomCursor(args['name'] as String);
+}
+
+Future<void> _checkDprChange(
+    TestFlutterView view, List<Map<dynamic, dynamic>> registrations) async {
+  final data = await _data();
+  final cursor = _CursorModel();
+  for (final dpr in [2.0, 1.0]) {
+    view.devicePixelRatio = dpr;
+    final session = buildCursorOfCache(cursor, 1, data).createSession(1);
+    await session.activate();
+    session.dispose();
+  }
+  expect(registrations, hasLength(2));
+  expect(registrations[0]['name'], isNot(registrations[1]['name']));
+  for (final args in registrations) {
+    await deleteCustomCursor(args['name'] as String);
+  }
+}