added the first T-API example - CamShift tracking
implemented OpenCL version of cv::convertScaleAbs
added OpenCL version of cv::scaleAdd
converted CPU-based example to T-API (Mat 2 UMat, etc)
[emscripten] Do not link to system libraries

This is not meaningful when compiling to javascript, and causes warning
at linking stage.
Merge pull request #2089 from ilya-lavrenov:tapi_examples
Merge pull request #2085 from severin-lemaignan:emscripten
added OpenCL version of cv::patchNaNs
Merge pull request #2091 from ilya-lavrenov:tapi_convertScaleAbs