* Add documentation for cvtColorTwoPlane

* Change brief and add links

* Add Shufflenet support in ONNX

* Add test for transpose layer

* cmake: don't ignore Python from PATH environment variable

- this breaks selection between 32/64-bit Python
- this breaks Anaconda/Conda environments
- it is not the CMake default behavior, expected by many projects

* cmake: add Python version check, fallback path on CMake 3.12+

* cmake: drop Python 2.6, allow version selection for Python 3.x

especially in CPU only processing

_tzcnt_u32() is not exported by clang-cl intrin.h so check for
clang-cl and enable an alterate for _tzcnt_u32()

Some discussions:
http://lists.llvm.org/pipermail/cfe-dev/2016-October/051329.html
https://bugs.llvm.org/show_bug.cgi?id=30506

TEST=Build with clang-cl

to avoid build break without `__OPENCV_BUILD`

  * if the src has odd number of height, access error happens
  * it could happen on width, too
  * check both dst and src range in both width and height

* fix #12012

* Replace CV_StsBadFlag with Error::StsBadFlag

Avoids passing -Wall, which will enable literally every warning in
Clang. Avoids passing -ffunction-sections etc to clang-cl, which each
generate warnings.

- to avoid multiple conditional calls via sub_group() functions

The `codec_tag` is only available when opening a file from disk. If `AVStream` is a network stream then `fourcc` must be obtained using `codec_id`. I have tested the following scenarios:
1) Open a `.mp4` file and verify that `codec_tag` is returned (old behavior)
2) Open a `rtsp` stream and verify that `codec_fourcc` is returned (Tested with a MJPEG, H264 and H265 stream)

  * avoid race condition

Update hog.cpp (#12649)