Merge 2.4 into master

Original pull requests:

* #2133 from apavlenko:2.4-perf_haar
* #2140 from Daniil-Osokin:fix_saving_untrained_svm_model
* #2143 from Daniil-Osokin:fix_docs_cvt_color_alpha_channel
* #2145 from andreasBihlmaier:2.4
* #2159 from SpecLad:1xN