fix: CameraComponent no longer throws Concurrent modification on stop

[lohnn]

Description

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.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

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[spydon]

Lgtm, just a minor comment on the test