Changes doc, images and sample code itself

Merge pull request #13803 from thangktran:thangktran/fix-misleading-runtime-assertion-calibrateCamera

objectPoints and imagePoints are not checked whether they're empty and
cause checkVector() to fail, thus result in a wrong error message.

Fixes: https://github.com/opencv/opencv/issues/6002

backport 997b7b18

* Add Sobel kernel which returns both dx and dy

* Splice dx and dy and extend add_border function

Also change some tests parameters

* Add borderValue parameter in test

* Introduces fluid kernel for sobelxy

Adds tests (basic and performance) on new backend

* Introduces BufHelper struct for some arithmetic

* Add GOut overloading for tuple of parameter pack

* Move getGOut_impl into namespace and add const to input references