libcamera: request: Make metadata_ a class instance

The metadata_ member variable is a pointer, for no specific reason.
Make it an instance and simplify the class constructor and destructor.

Suggested-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>

include/libcamera/internal/request.h

@@ -36,7 +36,7 @@ public:
 	Camera *camera() const { return camera_; }
 	bool hasPendingBuffers() const;
 
-	ControlList &metadata() { return *metadata_; }
+	ControlList &metadata() { return metadata_; }
 
 	bool completeBuffer(FrameBuffer *buffer);
 	void complete();
@@ -63,7 +63,7 @@ private:
 	std::unordered_set<FrameBuffer *> pending_;
 	std::map<FrameBuffer *, EventNotifier> notifiers_;
 	std::unique_ptr<Timer> timer_;
-	ControlList *metadata_;
+	ControlList metadata_;
 };
 
 } /* namespace libcamera */