cap_libv4l depends on an external library (libv4l) yet is still larger
(1966 loc vs 1822 loc).
It was initially introduced copy pasting cap_v4l in order to offload
various color conversions to libv4l.
However nowadays we handle most of the needed color conversions inside
OpenCV. Our own implementation is better tested and (probably) also
better performing. (as it can optionally leverage IPP/ OpenCL)

Currently cap_v4l is better maintained and generally the code is in
better shape. There is however an API
difference in getting unconverted frames:
* on cap_libv4l one need to set `CV_CAP_MODE_GRAY=1` or
`CV_CAP_MODE_YUYV=1`
* on cap_v4l one needs to set `CV_CAP_PROP_CONVERT_RGB=0`

the latter is more flexible though as it also allows accessing undecoded
JPEG images.

fixes #4563

docs: fix Kroeger2016 bib reference

move from opencv_contrib

[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

- cv::at => cv::Mat::at()
- removed unnecessary @ref

* Support for Matx read/write by FileStorage

* Only empty filestorage read now produces default Matx. Split Matx IO test into smaller units. Test checks for exception thrown if reading a Mat into a Matx of different size.

* moved DIS optical flow from opencv_contrib to opencv, moved TVL1 from opencv to opencv_contrib

* fixed compile warning

* TVL1 optical flow example moved to opencv_contrib

* significantly reduced OpenCV binary size by disabling IPP calls in some OpenCV functions: Sobel, Scharr, medianBlur, GaussianBlur, filter2D, mean, meanStdDev, norm, sum, minMaxIdx, sort.

* re-enable IPP in norm, since it's much faster (without adding too much space overhead)

* refined QRCodeDetector API for OpenCV 4.0

* expanded and tested QRCodeDetector::detectAndDecode()

* removed C API in the following modules: photo, video, imgcodecs, videoio

* trying to fix various compile errors and warnings on Windows and Linux

* continue to fix compile errors and warnings

* continue to fix compile errors, warnings, as well as the test failures

* trying to resolve compile warnings on Android

* Update cap_dc1394_v2.cpp

fix warning from the new GCC

G-API GPU-OpenCL backend (#13008)

* gpu/ocl backend core

* accuracy tests added and adjusted + license headers

* GPU perf. tests added; almost all adjusted to pass

* all tests adjusted and passed - ready for pull request

* missing license headers

* fix warning (workaround RGB2Gray)

* fix c++ magic

* precompiled header

* white spaces

* try to fix warning and blur test

* try to fix Blur perf tests

* more alignments with the latest cpu backend

* more gapi tests refactoring + 1 more UB issue fix + more informative tolerance exceed reports

* white space fix

* try workaround for SumTest

* GAPI_EXPORTS instead CV_EXPORTS

V4L (V4L2): Refactoring. Added missed camera properties. Fixed getting `INF` for some properties. Singlethread as always (#12893)

* cap_v4l:
1 Added cap_properties verbalization.
2 Set Get of properties elementary refactoring.
3 Removed converting parameters to/from [0,1] range.
4 Added all known conversion from V4L2_CID_* to CV_CAP_PROP_*

* cap_v4l:
1. Removed all query for parameters range.
2. Refactored capture initialization.
3. Added selecting input channel by CV_CAP_PROP_MODE. Default value -1 the channels not changed.

* cap_v4l:
1. Refactoring of Convert To RGB

* cap_v4l:
1. Fixed use of video buffer index.
2. Removed extra memcopy for grab image.
3. Removed device closing from autosetup_capture_mode_v4l2

* cap_v4l:
1. The `goto` was eliminated
2. Fixed use of temporary buffer index for V4L2_PIX_FMT_SN9C10X
3. Fixed use of the bufferIndex
4. Removed trailing spaces and unused variables.

* cap_v4l:
1. Alias for capture->buffers[capture->bufferIndex]
2. Reduced size of data for memcpy: bytesused instead of length
3. Refactoring. Code duplication. More info for debug

* cap_v4l:
1. Added the ability to grab and retrieveFrame independently several times

* cap_v4l:
1. Not need to close/open device for new capture parameters applying.
2. Removed using of device name as a flag that the capture is closed. Added sufficient function.
3. Refactoring. Added requestBuffers and createBuffers

* cap_v4l:
1. Added tryIoctl with `select` like was in mainloop_v4l2.
2. Fixed buffer request for device without closing the device.
3. Some static function moved to CvCaptureCAM_V4L
4. Removed unused defines

* cap_v4l:
1. Thread-safe now

* cap_v4l:
1. Fixed thread-safe destructor
2. Fixed FPS setting

* Missed brake

* Removed thread-safety

* cap_v4l:
1. Reverted conversion parameters to/from [0,1] by default for backward compatibility.
2. Added setting for turn off compatibility mode: set CV_CAP_PROP_MODE to 65536
3. Most static functions moved to CvCaptureCAM_V4L
4. Refactoring of icvRetrieveFrameCAM_V4L and using of frame_allocated flag

* cap_v4l:
1. Added conversion to RGB from NV12, NV21
2. Refactoring. Removed wrappers for known format conversions.

* Added `CAP_PROP_CHANNEL` to the enum VideoCaptureProperties.
CAP_V4L migrated to use VideoCaptureProperties.

* 1. Update comments.
2. Environment variable `OPENCV_VIDEOIO_V4L_RANGE_NORMALIZED` for setting default backward compatibility mode.
3. Revert getting of `CAP_PROP_MODE` as fourcc code in backward compatibility mode.

* videoio: update cap_v4l - compatibilityMode => normalizePropRange

* videoio(test): V4L2 MJPEG test

`v4l2-ctl --list-formats` should have 'MJPG' entry

* videoio: fix buffer initialization

to avoid "munmap: Invalid argument" messages

This reverts commit 7349b8f5 (partially).