fix: both flippers should respond to multi touch taps

lib/game/pinball_game.dart

@@ -77,7 +77,7 @@ class PinballGame extends PinballForge2DGame
     await super.onLoad();
   }
 
-  BoardSide? focusedBoardSide;
+  final focusedBoardSide = <int, BoardSide>{};
 
   @override
   void onTapDown(int pointerId, TapDownInfo info) {
@@ -90,9 +90,10 @@ class PinballGame extends PinballForge2DGame
         descendants().whereType<Plunger>().single.pullFor(2);
       } else {
         final leftSide = info.eventPosition.widget.x < canvasSize.x / 2;
-        focusedBoardSide = leftSide ? BoardSide.left : BoardSide.right;
+        focusedBoardSide[pointerId] =
+            leftSide ? BoardSide.left : BoardSide.right;
         final flippers = descendants().whereType<Flipper>().where((flipper) {
-          return flipper.side == focusedBoardSide;
+          return flipper.side == focusedBoardSide[pointerId];
         });
         flippers.first.moveUp();
       }
@@ -103,23 +104,23 @@ class PinballGame extends PinballForge2DGame
 
   @override
   void onTapUp(int pointerId, TapUpInfo info) {
-    _moveFlippersDown();
+    _moveFlippersDown(pointerId);
     super.onTapUp(pointerId, info);
   }
 
   @override
   void onTapCancel(int pointerId) {
-    _moveFlippersDown();
+    _moveFlippersDown(pointerId);
     super.onTapCancel(pointerId);
   }
 
-  void _moveFlippersDown() {
+  void _moveFlippersDown(int pointerId) {
-    if (focusedBoardSide != null) {
+    if (focusedBoardSide[pointerId] != null) {
       final flippers = descendants().whereType<Flipper>().where((flipper) {
-        return flipper.side == focusedBoardSide;
+        return flipper.side == focusedBoardSide[pointerId];
       });
       flippers.first.moveDown();
-      focusedBoardSide = null;
+      focusedBoardSide.remove(pointerId);
     }
   }
 }