Some `CV_Assert`s changed to `CV_Assert_N`s according to https://github.com/opencv/opencv/issues/12304

The while loop would run only once making it useless and leading
to confusion.

So, remove the unwanted while loop and just keep an infinite for
loop.

fixed call open(int id) failed

the static variables will cause race-condition when operating in
multithread scenarios.
Signed-off-by: Teng Yiliang <ylteng@outlook.com>
Signed-off-by: Teng Yiliang <yiliang.teng@weimob.com>

into cv::videoio_registry namespace

build fails with MSVS 2012 without additional flags

OpenCV binaries are compiled on Win10 environment

* Kinect2 support added to OpenNI2 backend

* more fixes for OpenNI2 backend

* whitespace fixed

* libfreenect2 support added

* const cast added

* mutable removed

Synchronized reading from camera with heavy frame processing
provides bad effects (huge frame latency, processing frames from the past).
Generally, there is no way to process each frame and some frames will be dropped.
Allow preventive frame dropping to reduce lag of processed frames.

This mode is applied to cameras only (opened by 'index').

Use Win32 TRUE/FALSE in SetUINT32() calls for bool parameters

`camid` value is always -1 after close().

- no behavior changes in grabFrame()
- added Lock2D() support into retrieveFrame()

Use "OPENCV_FFMPEG_USE_FIND_PACKAGE" variable to specify find_package() parameters

Using codec->time_base is deprecated to specify muxer settings.
Resolves issue with FPS value for AVI files with FFmpeg 4.0.

Related FFmpeg commits:
- https://github.com/FFmpeg/FFmpeg/commit/194be1f43ea391eb986732707435176e579265aa
- https://github.com/FFmpeg/FFmpeg/commit/91736025b2807995e29bd0661807c1c84f515fef