feat(flame): Set a default negative priority on the world for general…

… use (flame-engine#2572)

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

@@ -11,8 +11,15 @@ import 'package:vector_math/vector_math_64.dart';
 /// The primary feature of this component is that it disables regular rendering,
 /// and allows itself to be rendered through a [CameraComponent] only. The
 /// updates proceed through the world tree normally.
+///
+/// The [priority] of the world by default is the maximum 32bit negative int
+/// value to ensure it will always be earlier in the component tree than a
+/// [CameraComponent].
 class World extends Component implements CoordinateTransform {
-  World({super.children});
+  World({
+    super.children,
+    super.priority = -0x7fffffff,
+  });
 
   @override
   void renderTree(Canvas canvas) {}

packages/flame/test/camera/world_test.dart

@@ -0,0 +1,32 @@
+import 'package:flame/camera.dart';
+import 'package:flame/components.dart';
+import 'package:flame_test/flame_test.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('World', () {
+    testWithFlameGame(
+      'by default it has a negative max 32bit int priority',
+      (game) async {
+        final world = World()..addToParent(game);
+        final camera = CameraComponent(world: world)..addToParent(game);
+        await game.ready();
+
+        expect(world.priority, equals(-0x7fffffff));
+        expect(game.children, equals([world, camera]));
+      },
+    );
+
+    testWithFlameGame(
+      'with a custom priority putting it in front of the camera in the tree',
+      (game) async {
+        final world = World(priority: 4)..addToParent(game);
+        final camera = CameraComponent(world: world)..addToParent(game);
+        await game.ready();
+
+        expect(world.priority, equals(4));
+        expect(game.children, equals([camera, world]));
+      },
+    );
+  });
+}