libcamera: request: Move metadata_ to Private

Address a long standing \todo item that suggested to implement a
read-only interface for the Request::metadata() accessor and deflect to
the internal implementation for the read-write accessor used by pipeline
handlers.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Acked-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>

include/libcamera/internal/request.h

@@ -36,6 +36,8 @@ public:
 	Camera *camera() const { return camera_; }
 	bool hasPendingBuffers() const;
 
+	ControlList &metadata() { return *metadata_; }
+
 	bool completeBuffer(FrameBuffer *buffer);
 	void complete();
 	void cancel();
@@ -61,6 +63,7 @@ private:
 	std::unordered_set<FrameBuffer *> pending_;
 	std::map<FrameBuffer *, EventNotifier> notifiers_;
 	std::unique_ptr<Timer> timer_;
+	ControlList *metadata_;
 };
 
 } /* namespace libcamera */