Update window.cpp

modules/highgui/src/window.cpp

@@ -366,7 +366,7 @@ void cv::imshow( const String& winname, InputArray _img )
     if (useGl <= 0)
     {
         Mat img = _img.getMat();
-        CvMat c_img = img;
+        CvMat c_img = cvMat(img);
         cvShowImage(winname.c_str(), &c_img);
     }
     else