libcamera: camera_manager: Make the class a singleton

There can only be a single camera manager instance in the application.
Creating it as a singleton helps avoiding mistakes. It also allows the
camera manager to be used as a storage of global data, such as the
future event dispatcher.

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

include/libcamera/camera_manager.h

@@ -19,15 +19,19 @@ class PipelineHandler;
 class CameraManager
 {
 public:
-	CameraManager();
-
 	int start();
 	void stop();
 
 	std::vector<std::string> list() const;
 	Camera *get(const std::string &name);
 
+	static CameraManager *instance();
+
 private:
+	CameraManager();
+	CameraManager(const CameraManager &) = delete;
+	void operator=(const CameraManager &) = delete;
+
 	DeviceEnumerator *enumerator_;
 	std::vector<PipelineHandler *> pipes_;
 };