scene_viewer: Use get_single_mut as there should be exactly one CameraController

Author: superdump

examples/tools/scene_viewer.rs

@@ -342,15 +342,15 @@ fn camera_controller(
         mouse_delta += mouse_event.delta;
     }
 
-    for (mut transform, mut options) in query.iter_mut() {
+    if let Ok((mut transform, mut options)) = query.get_single_mut() {
         if !options.initialized {
             let (_roll, yaw, pitch) = transform.rotation.to_euler(EulerRot::ZYX);
             options.yaw = yaw;
             options.pitch = pitch;
             options.initialized = true;
         }
         if !options.enabled {
-            continue;
+            return;
         }
 
         // Handle key input