fix some errors found by static analyzer. (#12391)
* fix possible divided by zero and by negative values
* only 4 elements are used in these arrays
* fix uninitialized member
* use boolean type for semantic boolean variables
* avoid invalid array index
* to avoid exception and because base64_beg is only used in this block
* use std::atomic<bool> to avoid thread control race condition
OpenCV pthreads-based implementation changes:
- rework worker threads pool, allow to execute job by the main thread too
- rework synchronization scheme (wait for job completion, threads 'pong' answer is not required)
- allow "active wait" (spin) by worker threads and by the main thread
- use _mm_pause() during active wait (support for Hyper-Threading technology)
- use sched_yield() to avoid preemption of still working other workers
- don't use getTickCount()
- optional builtin thread pool profiler (disabled by compilation flag)
