add overlay to scanner

@@ -62,6 +62,10 @@ class MobileScanner extends StatefulWidget {
   /// Default: false
   final bool startDelay;
 
+  /// The overlay which will be painted above the scanner when has started successful.
+  /// Will no be pointed when an error occurs or the scanner hasn't be started yet.
+  final Widget? overlay;
+
   /// Create a new [MobileScanner] using the provided [controller]
   /// and [onBarcodeDetected] callback.
   const MobileScanner({
@@ -74,6 +78,7 @@ class MobileScanner extends StatefulWidget {
     this.placeholderBuilder,
     this.scanWindow,
     this.startDelay = false,
+    this.overlay,
     super.key,
   });
 
@@ -252,7 +257,10 @@ class _MobileScannerState extends State<MobileScanner>
               _controller.updateScanWindow(scanWindow);
             }
 
-            return ClipRect(
+            return Stack(
+              alignment: Alignment.center,
+              children: [
+                ClipRect(
                   child: LayoutBuilder(
                     builder: (_, constraints) {
                       return SizedBox.fromSize(
@@ -270,6 +278,10 @@ class _MobileScannerState extends State<MobileScanner>
                       );
                     },
                   ),
+                ),
+                if (widget.overlay != null)
+                  widget.overlay!
+              ],
             );
           },
         );