Merge 2.4 into master

Original pull requests:

* #2541 from JinoBetti:2.4
* #2548 from jet47:gpu_mog2_test
* #2551 from ilya-lavrenov:ocl_nonfree
* #2553 from ilya-lavrenov:typo
* #2554 from asmorkalov:ocv_249_version
* #2572 from jet47:gpu-generalized-hough