fix: CameraComponent no longer throws Concurrent modification on stop (…

…#2997)

When camera is following a component and trying to follow another target
I got an exception:
```
dart:core                                              Iterable.forEach
package:flame/src/camera/camera_component.dart 326:25  CameraComponent.stop
package:flame/src/camera/camera_component.dart 309:5   CameraComponent.follow
test/camera/camera_component_test.dart 69:14           main.<fn>.<fn>
package:flame_test/src/test_flame_game.dart 80:21      testWithGame.<fn>

Concurrent modification during iteration: _Set len:0.
```

Copying viewfinder children before iterating through it and removing
child from parent does the trick, as we are not iterating through the
same list as we are removing items from, but rather a copy of it.

I tried to use camera from `game` provided in the test but it is AFAIK
not mounted and will hence not queue modifications (adds and removes).
Hence I create a new camera and mount it.

.github/.cspell/gamedev_dictionary.txt

@@ -46,6 +46,7 @@ raytracing # rendering techniques that calculates light rays as straight lines
 rects # plural of rect
 respawned # past tense of respawn
 respawn # when the player character dies and is brought back after some time and penalties
+retarget # to direct (something) toward a different target
 RGBA # red green blue alpha
 RGBO # red green blue opacity
 rrect # rounded rect

packages/flame/lib/src/camera/camera_component.dart

@@ -323,7 +323,7 @@ class CameraComponent extends Component {
 
   /// Removes all movement effects or behaviors from the viewfinder.
   void stop() {
-    viewfinder.children.forEach((child) {
+    viewfinder.children.toList().forEach((child) {
       if (child is FollowBehavior || child is MoveEffect) {
         child.removeFromParent();
       }

packages/flame/test/camera/camera_component_test.dart

@@ -60,6 +60,20 @@ void main() {
       }
     });
 
+    testWithFlameGame('camera should be able to retarget follow', (game) async {
+      // Creating new camera as the one included with game is not mounted and
+      // will therefore not be queued.
+      final camera = CameraComponent(world: game.world)..addToParent(game);
+      final player = PositionComponent()..addToParent(game.world);
+      final player2 = PositionComponent()..addToParent(game.world);
+      camera.follow(player);
+      camera.follow(player2);
+      await game.ready();
+
+      expect(camera.viewfinder.children.length, 1);
+      expect(camera.viewfinder.children.first, isA<FollowBehavior>());
+    });
+
     testWithFlameGame('follow with snap', (game) async {
       final world = World()..addToParent(game);
       final player = PositionComponent()