Fixed build for samples/cpp/train_HOG.cpp.

get_hogdescriptor_visu is passed img->clone(), which is not usable
as a non-const reference (and it doesn't modify the argument, anyway).
Added returnDFVal parameter to parallel CvSVM::predict function
implemented OpenCL version of cv::accumulate****
Merge pull request #2214 from cosminBoaca:add_returnDFVal_parallel_predict
Merge pull request #2212 from SpecLad:hog-sample-const-mat
added cv::calcHist to T-API (only for CV_8UC1 with 256 bins)
added performance test
added performance test
added conditional compilation
Merge pull request #2226 from ilya-lavrenov:tapi_calcHist
Merge pull request #2220 from ilya-lavrenov:tapi_accumulate