This fixes a compiler error on MSVC with Unreal Engine (at least).

backport of commit: 4e998c9a

This fixes a compiler error on MSVC with Unreal Engine (at least).

Keep a local copy of `features` from `flann::GenericIndex ` constructor (#14898)

* Fix warnings on Windows

* Fix #4376

When running with >1 OpenCV thread, KMeans index generation was
non-deterministic because of a RWW race. Issue is resolved by removing
the offending logic from the parallel section.

All <arm_neon.h> includes in core/cv_cpu_dispatch.h are protected by an
ifndef __CUDACC__ to prevent attempting to use neon intrinsics when
compiling cuda kernels (.cu) -- this prevents hard errors such as
  error: identifier "__builtin_neon_qi" is undefined

Add this same protection to flann/dist.h to fix compilation involving
flann.hpp.

* Document distance functors in dist.h

* Add spec for Distance

* Generate appropriate links for symbols

* core: drop usage of CV_USRTYPE1 in OpenCV

avoid OpenCV crashes due size change CV_ELEM_SIZE(CV_USRTYPE1): 8 -> 2

* ! fix persistence internal types

* 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

Found via `codespell`

- removed tr1 usage (dropped in C++17)
- moved includes of vector/map/iostream/limits into ts.hpp
- require opencv_test + anonymous namespace (added compile check)
- fixed norm() usage (must be from cvtest::norm for checks) and other conflict functions
- added missing license headers

Update miniflann.cpp

Update test_stereomatching.cpp

size_t is not supported cv::Mat type

Remove unnecessary Non-ASCII characters from source code (#9075)

* Remove unnecessary Non-ASCII characters from source code

Remove unnecessary Non-ASCII characters and replace them with ASCII
characters

* Remove dashes in the @param statement

Remove dashes and place single space in the @param statement to keep
coding style

* misc: more fixes for non-ASCII symbols

* misc: fix non-ASCII symbol in CMake file

Add missing sstream header for decl of std::ostringstream in line 104

std::rand() has no thread-safe guarantee.