Add asset for number of cameras (should be 1 per display state)

src/game.rs

@@ -28,9 +28,9 @@ impl Plugin for GamePlugin {
             .add_systems(
                 PreUpdate,
                 (
-                    asserts::<display2d::Display2d>,
-                    asserts::<display3d::Display3d>,
-                ),
+                    asserts::<display2d::Display2d>.run_if(in_state(DisplayState::Display2d)),
+                    asserts::<display3d::Display3d>.run_if(in_state(DisplayState::Display3d)),
+                ).run_if(in_state(GameState::Play)),
             )
             .add_systems(
                 PostUpdate,
@@ -489,6 +489,7 @@ fn capture_piece(mut events: Query<&mut Visibility, Added<Captured>>) {
 fn asserts<T: Component>(
     selected_pieces: Query<Entity, (With<Piece>, With<T>, With<Selected>)>,
     selected_tiles: Query<Entity, (With<Tile>, With<T>, With<Selected>)>,
+    cameras: Query<Entity, (With<Camera>, With<T>)>,
 ) {
     if selected_pieces.iter().len() > 2 {
         selected_pieces.iter().for_each(|e| {
@@ -505,4 +506,7 @@ fn asserts<T: Component>(
             selected_tiles.iter().len()
         );
     }
+    if cameras.iter().len() != 1 {
+        panic!("There should be 2 cameras in this state, but there is only {}", cameras.iter().len());
+    }
 }