Use num_threads clause of #pragma omp parallel instead to avoid calling omp_set_num_threads for the entire application

Aravis: Use of std::fabs, added support for 16-bit mono files and exposure compensation (#8711)

* Use of std::fabs, added support for 16-bit mono files

* Correction in priority2 stage &  adding exposure compensation

issue 8723: Changed doc for cv::imwrite to reflect correct default flag used for png compression strategy.

* Extending template_matching tutorial with Java

* adding mask to java version of the tutorial

* adding the python toggle and code

* updating table of content

* adding py and java to table of content

* adding mask to python

* going back to markdown with duplicated text

* non duplicated text

Avoid shifting by 32bits; undefined behavior according to c - standard

Fix OpenCL's wrong output of calcOpticalFlowPyrLK function's output vector of err. Improve err's precison of the calcOpticalFlowPyrLK OpenCL function and add the relative test.