fix(flutter): size unzoomed remote cursors to the local system

flutter/lib/utils/cursor_size.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+import 'package:image/image.dart' as img;
+
+int cursorVisibleSize(img.Image image) {
+  var left = image.width;
+  var top = image.height;
+  var right = -1;
+  var bottom = -1;
+  for (final pixel in image) {
+    if (pixel.a == 0) continue;
+    if (pixel.x < left) left = pixel.x;
+    if (pixel.x > right) right = pixel.x;
+    if (pixel.y < top) top = pixel.y;
+    if (pixel.y > bottom) bottom = pixel.y;
+  }
+  if (right < left) return 0;
+  final width = right - left + 1;
+  final height = bottom - top + 1;
+  return width > height ? width : height;
+}
+
+class LocalCursorSize extends ValueNotifier<double?> {
+  LocalCursorSize() : super(null);
+
+  static const _channel = MethodChannel('org.rustdesk.rustdesk/cursor');
+  double? _devicePixelRatio;
+  int _generation = 0;
+
+  void ensureLoaded(double devicePixelRatio) {
+    if (_devicePixelRatio == devicePixelRatio) return;
+    _devicePixelRatio = devicePixelRatio;
+    refresh();
+  }
+
+  Future<void> refresh() async {
+    final generation = ++_generation;
+    try {
+      final size = await _channel.invokeMethod<double>('getSystemCursorSize');
+      if (size == null || !size.isFinite || size <= 0) {
+        throw const FormatException('Invalid local system cursor size');
+      }
+      if (generation == _generation) value = size;
+    } catch (error, stack) {
+      FlutterError.reportError(FlutterErrorDetails(
+          exception: error, stack: stack, library: 'local cursor size'));
+    }
+  }
+
+  @override
+  void dispose() {
+    ++_generation;
+    super.dispose();
+  }
+}