Leave passed in matrix types alone if disparity image is not empty

This preserves the behavior documented at the time the change was made.  The sample gpu stereo program ceased to work for BP and CSBP as it started returning a `CV_16SC1` matrix, which showed up as gray as all disparities were near zero.  Further, the text drawn on would not show for the same reason.  I also modify the sample code to convert to `CV_8UC1` explicitly, should behaviour change again.