* Fixed compilation error WITH_OPENMP under Windows

* Move up OpenMP condition

* simplify condition

* dims must > 0 or latter sz[dims-1] will underflow

* rewrote Mat::convertTo() and convertScaleAbs() to wide universal intrinsics; added always-available and SIMD-optimized FP16<=>FP32 conversion

* fixed compile warnings

* fix some more compile errors

* slightly relaxed accuracy threshold for int->float conversion (since we now do it using single-precision arithmetics, not double-precision)

* fixed compile errors on iOS, Android and in the baseline C++ version (intrin_cpp.hpp)

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

POD structures can't have constructors.

Signed-off-by: Zhenqing Hu <huzq85@gmail.com>

Commits:
- 09837928
- 10fb88d0

Excluded changes with std::atomic (C++98 requirement)

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

- support passing of `std::vector<KeyPoint>` via InputArray

* fix some static analyzer warnings

* fix some static analyzer warnings

* fix race condition of workthread control

to forbid Mat1f

Checks are not reliable: empty uninitialized `cv::Mat` has `CV_8UC1` type

* Add HPX backend for OpenCV implementation
Adds hpx backend for cv::parallel_for_() calls respecting the nstripes chunking parameter. C++ code for the backend is added to modules/core/parallel.cpp. Also, the necessary changes to cmake files are introduced.
Backend can operate in 2 versions (selectable by cmake build option WITH_HPX_STARTSTOP): hpx (runtime always on) and hpx_startstop (start and stop the backend for each cv::parallel_for_() call)

* WIP: Conditionally include hpx_main.hpp to tests in core module
Header hpx_main.hpp is included to both core/perf/perf_main.cpp and core/test/test_main.cpp.
The changes to cmake files for linking hpx library to above mentioned test executalbles are proposed but have issues.

* Add coditional iclusion of hpx_main.hpp to cpp cpu modules

* Remove start/stop version of hpx backend

Multi-argument CV_Assert() is accessible via CV_Assert_N() (with malformed messages).

Added to CPU path only.
OpenCL code path still expects NV12 only (according to Intel OpenCL extension)

To avoid duplicates in binaries

* core(test): merge hang test

* core(merge/split): fix intrin optimization

- use '3.4.x' cache name for current maintenance series (there are no serious changes between releases)
- message is shown only once during creation of new cache directory
- use OPENCV_CACHE_SHOW_CLEANUP_MESSAGE=0 to hide this warning

Fix for SSE2 intrinsics problem in the part of saturation arithmetic processing during 32s->16u packed conversion -
for some big negative values less than -INT_MAX+32767 the sign of the numbers is lost due to overflow that leads to
incorrect saturation to MAX value, instead of zero.
The issue is not reproduced with CV_ENABLED_INTRINSICS=OFF

Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`

- check result from CV_XADD() directly
- decrease urefcount after unmap() call only

Changed the type of variable *r* from int to size_t.
This change makes sure that a valid result of std::max(r + delta,
(r*3+1)/2) can be passed into the reserve function.

to avoid silent crashes of OpenCL worker threads.

* 1. changed static const __m128/256 to const __m128/256 to avoid wierd instructions and calls inserted by compiler.
2. added universal intrinsics that wrap MOVNTPS and other such (non-temporary or "no cache" store) instructions. v_store_interleave() and v_store() got respective flags/overloaded variants
3. rewrote split & merge to use the "no cache" store instructions. It resulted in dramatic performance improvement when processing big arrays

* hopefully, fixed some test failures where 4-channel v_store_interleave() is used

* added missing implementation of the new universal intrinsics (v_store_aligned_nocache() etc.)

* fixed silly typo in the new intrinsics in intrin_vsx.hpp

* still trying to fix VSX compiler errors

* still trying to fix VSX compiler errors

* still trying to fix VSX compiler errors

* still trying to fix VSX compiler errors

* fixed/updated v_load_deinterleave and v_store_interleave intrinsics; modified split() and merge() functions to use those intrinsics

* fixed a few compile errors and bug in v_load_deinterleave(ptr, v_uint32x4& a, v_uint32x4& b)

* fixed few more compile errors

* core: FileStorage - add support for writing vector<String> with bindings

* python: extend persistence test for string sequences

* core:OE-27 prepare universal intrinsics to expand (#11022)

* core:OE-27 prepare universal intrinsics to expand (#11022)

* core: Add universal intrinsics for AVX2

* updated implementation of wide univ. intrinsics; converted several OpenCV HAL functions: sqrt, invsqrt, magnitude, phase, exp to the wide universal intrinsics.

* converted log to universal intrinsics; cleaned up the code a bit; added v_lut_deinterleave intrinsics.

* core: Add universal intrinsics for AVX2

* fixed multiple compile errors

* fixed many more compile errors and hopefully some test failures

* fixed some more compile errors

* temporarily disabled IPP to debug exp & log; hopefully fixed Doxygen complains

* fixed some more compile errors

* fixed v_store(short*, v_float16&) signatures

* trying to fix the test failures on Linux

* fixed some issues found by alalek

* restored IPP optimization after the patch with AVX wide intrinsics has been properly tested

* restored IPP optimization after the patch with AVX wide intrinsics has been properly tested