fix: Disallow mutatation of SpriteGroupComponent.sprites

packages/flame/lib/src/components/input/sprite_button_component.dart

@@ -47,12 +47,12 @@ class SpriteButtonComponent extends SpriteGroupComponent<ButtonState>
 
   set button(Sprite value) {
     _button = value;
-    sprites?[ButtonState.up] = value;
+    updateSprite(ButtonState.up, value);
   }
 
   set buttonDown(Sprite value) {
     _buttonDown = value;
-    sprites?[ButtonState.down] = value;
+    updateSprite(ButtonState.down, value);
   }
 
   @override

packages/flame/lib/src/components/sprite_animation_group_component.dart

@@ -153,7 +153,7 @@ class SpriteAnimationGroupComponent<T> extends PositionComponent
   /// Returns the map of animation state and their corresponding animations.
   ///
   /// If you want to change the contents of the map use the animations setter
-  /// and pass in a new map of animations
+  /// and pass in a new map of animations.
   Map<T, SpriteAnimation>? get animations =>
       _animations != null ? Map.unmodifiable(_animations!) : null;
 

packages/flame/lib/src/components/sprite_group_component.dart

@@ -80,7 +80,12 @@ class SpriteGroupComponent<T> extends PositionComponent
   bool get autoResize => _autoResize;
 
   /// Returns the sprites map.
-  Map<T, Sprite>? get sprites => _sprites;
+  ///
+  /// If you want to change the contents of the map use the sprites setter
+  /// and pass in a new map of sprites, or use [updateSprite] to update a
+  /// specific sprite.
+  Map<T, Sprite>? get sprites =>
+      _sprites != null ? Map.unmodifiable(_sprites!) : null;
 
   /// Sets the given [value] as sprites map.
   set sprites(Map<T, Sprite>? value) {
@@ -90,6 +95,12 @@ class SpriteGroupComponent<T> extends PositionComponent
     }
   }
 
+  /// Updates the sprite for the given key.
+  void updateSprite(T key, Sprite sprite) {
+    _sprites![key] = sprite;
+    _resizeToSprite();
+  }
+
   /// Sets the given value of autoResize flag.
   ///
   /// Will update the [size] to fit srcSize of