medianBlur called with "empty" source and ksize >= 7 crashes application with accessviolation. With this extra assert this is avoided and the application may normally catch the thrown exception.

 - brush up the implementation

* fix typos.

* Fix typos.

* Fix typos.

* Fix typos.

* Fix typos.

Now a user can use `std::cout` to print an object of `cv::UMat` just like `cv::Mat`.

 - add more channels/depth performance test for blend

* Fix the documentation for Mat::diag(int).

Fix issue #8181

* Fix the documentation for Mat::diag(int).

Fix issue #8181.

* Add support for printing out cv::Complex.

* Remove extra spaces.

* cv::Complex is submitted as a new pull request.

Instead of CL_DEVICE_EXTENSIONS in Querying OpenCLVersion, it should be CL_DEVICE_VERSION flag which needs to be passed to the function.

Add support for printing out cv::Complex. (#8208)

* Add support for printing out cv::Complex.

* Conform to the format of std::complex.

* Remove extra spaces.

* Remove extra spaces.