libcamera: camera_manager: Return a copy of the vector from cameras()

Making CameraManager::cameras() thread-safe requires returning a copy of
the cameras vector instead of a reference. This is also required for
hot-plugging support and is thus desirable.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

include/libcamera/camera_manager.h

@@ -30,7 +30,7 @@ public:
 	int start();
 	void stop();
 
-	const std::vector<std::shared_ptr<Camera>> &cameras() const;
+	std::vector<std::shared_ptr<Camera>> cameras() const;
 	std::shared_ptr<Camera> get(const std::string &name);
 	std::shared_ptr<Camera> get(dev_t devnum);