Old flag resulted in software implementation being selected when the Intel decoder is not the primary adapter.

- exception safe smart pointers, fixed many memory leaks
- fixed errors handling
- fixed strings handling
- fixed file existence check
- fixed toFraction() implementation
- add 'drop=true' for v4l2 input
- avoid overriding custom pipelines sink's 'max-buffers' value
- updated valgrind supression rules
- added support for GStreamer native leaks tracking (via gst_deinit() call)
- repaired capturing through GStreamer0.10 (fixed CV_8UC1)

causes leaks if FFmpeg is disabled through OPENCV_VIDEOIO_PRIORITY_FFMPEG=0

VideoCapture Direct Show. Added getter for CV_CAP_PROP_CONVERT_RGB, CAP_PROP_CHANNEL properties. (#14592)

* Added getter for CV_CAP_PROP_CONVERT_RGB, CAP_PROP_CHANNEL properties.
Some refactoring.

* One space less.

- drop legacy check_module() macro
- drop 'ALIASOF_'

* macosx: fix C++11 build

backporting commit c3cf35ab

* macosx: eliminate build warning

Do not use compound literals (fix MSVC C4576) (#14276)

* do not use compound literals (compat cpp98)

* only fix msvc path

* remove unnecessary cplusplus check

According to the gstreamer docs [1], the GstMessage pointer returned by
gst_bus_pop() is nullable, meaning NULL is a valid return value.

Previously, gst_is_missing_plugin_message would throw an assert when its
message object parameter would fail the GST_IS_MESSAGE macro check,
crashing the entire process (unless running in a try-catch block of course).

Instead of relying on valid messages, check if the message object itself is
valid before passing it to other gstreamer functions.

[1] https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstBus.html#gst-bus-popSigned-off-by: Christopher N. Hesse <raymanfx@gmail.com>

- V4L2_CID_MPEG_VIDEO_H264_VUI_EXT_SAR_HEIGHT
- V4L2_CID_MPEG_VIDEO_H264_VUI_EXT_SAR_WIDTH

- better handling of enum arguments
- less merge conflicts

* Enabled -Wnarrowing warning

* Fixed type narrowing issues

* Cast python constants

* Use long long for python constants

* Use int for python constants with fallback to long

* Update cv2.cpp

backport 997b7b18

On windows 7, MinCore_Downlevel does not work correctly. However, the only API used was QISearch, which can be found in Shlwapi.

Closes #12010

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