Fix scanner overlay examples

Author: IanVS

example/lib/barcode_scanner_window.dart

@@ -131,15 +131,15 @@ class ScannerOverlay extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    // TODO: use `Offset.zero & size` instead of Rect.largest
     // we need to pass the size to the custom paint widget
-    final backgroundPath = Path()..addRect(Rect.largest);
+    final backgroundPath = Path()
+      ..addRect(Rect.fromLTWH(0, 0, size.width, size.height));
     final cutoutPath = Path()..addRect(scanWindow);
 
     final backgroundPaint = Paint()
       ..color = Colors.black.withOpacity(0.5)
       ..style = PaintingStyle.fill
-      ..blendMode = BlendMode.dstOut;
+      ..blendMode = BlendMode.dstOver;
 
     final backgroundWithCutout = Path.combine(
       PathOperation.difference,

example/lib/mobile_scanner_overlay.dart

@@ -102,9 +102,9 @@ class ScannerOverlay extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    // TODO: use `Offset.zero & size` instead of Rect.largest
     // we need to pass the size to the custom paint widget
-    final backgroundPath = Path()..addRect(Rect.largest);
+    final backgroundPath = Path()
+      ..addRect(Rect.fromLTWH(0, 0, size.width, size.height));
 
     final cutoutPath = Path()
       ..addRRect(
@@ -120,7 +120,7 @@ class ScannerOverlay extends CustomPainter {
     final backgroundPaint = Paint()
       ..color = Colors.black.withOpacity(0.5)
       ..style = PaintingStyle.fill
-      ..blendMode = BlendMode.dstOut;
+      ..blendMode = BlendMode.dstOver;
 
     final backgroundWithCutout = Path.combine(
       PathOperation.difference,