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.