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?

Interesting, it seems `CL_FLOAT` is not supported with `CL_BGRA` but is supported with `CL_RGBA` (see https://www.khronos.org/registry/cl/sdk/2.0/docs/man/xhtml/cl_image_format.html). I'd update this to use `CL_RGBA` (as well as the tests) and it should work. We'd just have to now make sure that when we copy a `CV_32FC4` `cv::Mat` to an `image2d` that we swap the components around (with `cvtColor` or do it on the device, it would probably be good to have a generic kernel for this).