fix: Disallow mutatation of SpriteGroupComponent.sprites (#3185)

Many users try to modify the `SpriteGroupComponent.sprites` map, but
that doesn't work since it has to go through the setter, so this PR
makes the returned map from the getter unmodifiable.
You can either set it by using the setter or set a particular sprite by
using the newly created `updateSprite` method.

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