* Fix drawMatches and drawKeypoints for BGRA input images

* keep BGRA too

* Combine different input types for drawMatches test

* Get/Set functions for BRISK parameters, issue #11527.

Allows setting threshold and octaves parameters after creating a brisk object. These parameters do not affect the initial pattern initialization and can be changed later without re-initialization.

* Fix doc parameter name.

* Brisk get/set functions tests. Check for correct value and make tests independent of default parameter values.

* Add dummy implementations for BRISK get/set functions not to break API in case someone has overloaded the Feature2d::BRISK interface. This makes BRISK different from the other detectors/descriptors on the other hand, where get/set functions are pure virtual in the interface.

[evolution] Stitching for OpenCV 4.0

* stitching: wrap Stitcher::create for bindings

* provide method for consistent stitcher usage across languages

* samples: add python stitching sample

* port cpp stitching sample to python

* stitching: consolidate Stitcher create methods

* remove Stitcher::createDefault, it returns Stitcher, not Ptr<Stitcher> -> inconsistent API
* deprecate cv::createStitcher and cv::createStitcherScans in favor of Stitcher::create

* stitching: avoid anonymous enum in Stitcher

* ORIG_RESOL should be double
* add documentatiton

* stitching: improve documentation in Stitcher

* stitching: expose estimator in Stitcher

* remove ABI hack

* stitching: drop try_use_gpu flag

* OCL will be used automatically through T-API in OCL-enable paths
* CUDA won't be used unless user sets CUDA-enabled classes manually

* stitching: drop FeaturesFinder

* use Feature2D instead of FeaturesFinder
* interoperability with features2d module
* detach from dependency on xfeatures2d

* features2d: fix compute and detect to work with UMat vectors

* correctly pass UMats as UMats to allow OCL paths
* support vector of UMats as output arg

* stitching: use nearest interpolation for resizing masks

* fix warnings

* integrated the new C++ persistence; removed old persistence; most of OpenCV compiles fine! the tests have not been run yet

* fixed multiple bugs in the new C++ persistence

* fixed raw size of the parsed empty sequences

* [temporarily] excluded obsolete applications traincascade and createsamples from build

* fixed several compiler warnings and multiple test failures

* undo changes in cocoa window rendering (that was fixed in another PR)

* fixed more compile warnings and the remaining test failures (hopefully)

* trying to fix the last little warning

Relevant guards can be found in
https://github.com/opencv/opencv/blob/ef5579dc8667e5eb5e149acc4af898421eed99da/modules/features2d/src/kaze/AKAZEConfig.h#L8
and
https://github.com/opencv/opencv/blob/ef5579dc8667e5eb5e149acc4af898421eed99da/modules/ml/include/opencv2/ml.hpp#L44

* Cleanup macros and enable expansion of `__VA_ARGS__` for Visual Studio

* Macros for enum-arguments backwards compatibility

* Convert struct Param to enum struct

* Enabled ParamType.type for enum types

* Enabled `cv.read` and `cv.write` for enum types

* Rename unnamed enum to AAKAZE.DescriptorType

* Rename unnamed enum to AccessFlag

* Rename unnamed enum to AgastFeatureDetector.DetectorType

* Convert struct DrawMatchesFlags to enum struct

* Rename unnamed enum to FastFeatureDetector.DetectorType

* Rename unnamed enum to Formatter.FormatType

* Rename unnamed enum to HOGDescriptor.HistogramNormType

* Rename unnamed enum to DescriptorMatcher.MatcherType

* Rename unnamed enum to KAZE.DiffusivityType

* Rename unnamed enum to ORB.ScoreType

* Rename unnamed enum to UMatData.MemoryFlag

* Rename unnamed enum to _InputArray.KindFlag

* Rename unnamed enum to _OutputArray.DepthMask

* Convert normType enums to static const NormTypes

* Avoid conflicts with ElemType

* Rename unnamed enum to DescriptorStorageFormat

* 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

to share with opencv_contrib/xfeatures2d

* 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 -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`

fixes handling of empty matrices in some functions (#11634)

* a part of PR #11416 by Yuki Takehara

* moved the empty mat check in Mat::copyTo()

* fixed some test failures

Bug was added in f6ceeaa2 commit, different angle computation functions have different parameter order.

To investigate sporadic failures of Features2d_FLANN_Auto.regression test.

Found via `codespell`