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
Merge pull request #2214 from cosminBoaca:add_returnDFVal_parallel_predict
Merge pull request #2212 from SpecLad:hog-sample-const-mat