Merge 2.4 into master

Original pull requests:

* #1959 from botverse:2.4
* #1968 from pentschev:fix_indentation_freak_2.4
* #1975 from magomimmo:clj-tut
* #1976 from alalek:ocl_multiple_command_queues
* #1980 from alalek:ocl_interop_initialization
* #1990 from AD-530:Issue3164