Limit the maximal number of threads

[luowyang]

VideoCap::open() sets the number of threads to whatever is returned by std::thread::hardware_concurrency() in this line. However, on machines with many cores the number of threads will exceed the maximum recommended by FFmpeg (which is 16 as defined here). This may degrade performance, and FFmpeg will issue an annoying warning says "Using a thread count greater than 16 is not recommended". It may be wiser to follow the recommendation and limit the number of threads to no larger than 16.

[LukasBommes]

Thanks for the contribution @luowyang. I will include this with the next release.