added histogram example based on opencv

Task reference #222

I am not able to have a consistent results with following code.
Can you have a look on this?

Yeah, this looks correct. If you have extra time it would be good to add a test for this format (in `test/test_interop_opencv.cpp`). Something simple like creating small (e.g. 2x2) `cv::Mat` with CV_32FC4, copying it to a `image2d` on the device, then copying it back and verifying the image data remains the same.