VideoCapture: copy the captured frame, to avoid dangling Mats

Previously, VideoCapture::retrieve would return a Mat that referenced
the internal IplImage. Since the latter is rewritten every time a
frame is captured, it means that if the user captures two frames in a row,
the first frame would reference nothing. Similar if a user captures a frame,
then destroys the VideoCapture instance.

Note that the other branch of the if isn't affected, since flip allocates
a new Mat.

modules/highgui/src/cap.cpp

@@ -523,7 +523,7 @@ bool VideoCapture::retrieve(Mat& image, int channel)
         return false;
     }
     if(_img->origin == IPL_ORIGIN_TL)
-        image = Mat(_img);
+        Mat(_img).copyTo(image);
     else
     {
         Mat temp(_img);