taking cv::mat const ref in frame constructor

src/common_class/frame.h

@@ -48,7 +48,7 @@ public:
 	/**
 	 * \brief Constructor
 	 */
-	Frame(const cv::Mat _img, const int& _id = 0, const KeyPointVector _kps = KeyPointVector(), const cv::Mat _desc = cv::Mat()) :
+	Frame(const cv::Mat& _img, const int& _id = 0, const KeyPointVector _kps = KeyPointVector(), const cv::Mat _desc = cv::Mat()) :
 		id_(_id),
 		img_(_img),
 		kps_(_kps),