Previously, VideoCapture::retrieve would return a Mat that referenced
the internal IplImage. Since the latter is rewritten every time a
frame is captured, it means that if the user captures two frames in a row,
the first frame would reference nothing. Similar if a user captures a frame,
then destroys the VideoCapture instance.

Note that the other branch of the if isn't affected, since flip allocates
a new Mat.

VideoCapture didn't call release method and just ignored the new capture sources.
OpenCV documentation:
  bool VideoCapture::open(const string& filename);
  bool VideoCapture::open(int device);

The methods first call VideoCapture::release() to close the already opened file or camera.

Bug in Video for Windows capture init fixed;
Media Foundation based capture finalization fixed;
Highgui tests for video i/o updated.

Set and Get methods are not implemented;
Camera based video capture is broken due to modifications.

Code formating fixed;
GrabFrame method implemented correclty.

TIFF build was fixed;
Direct Show back-end was enabled;
More accurate guards implemented.

GPU module turned off;
TIFF support turned off;
Windows Managment and Video IO (VFW) turned off.

* Decouple Video4Linux2 support from Video4Linux as existence of
  v4l2 on a system does not imply support for v4l.
* Don't use V4L's struct video_window in V4L2 code.
* Removed __USE_GNU as comment says:
      /* support for MJPEG is only available with libjpeg and gcc,
         because it's use libjepg and fmemopen()
  so replace with test for fmemopen() if found necessary.

The video(4) driver provides a Video4Linux2 compatible API for
various video peripherals. This patch propagates HAVE_VIDEOIO if
the sys/videoio.h include file is found, which is the signature of
video(4).

Camera connect->disconnect->connect leads to sigsegv.
Apply properties method reimplemented.
(cherry picked from commit 710e0709)

Camera connect->disconnect->connect leads to sigsegv.
Apply properties method reimplemented.

reverted the original VideoCapture::read() code; moved frame index check to where it should belong: ffmpeg wrapper; applied patch by Rob Reilink (ticket #1971)

added explicit VideoWriter::release(). Fixed video stream finalization (write remaining frames); Fixed occasional crash in the case of odd-width or odd-height videos; Fixed a few problems in ffmpeg tests. Positioning still does not work well.

modified highgui and ts modules and cmakelists for ios cross-compiling, added ios toolchains, added ios build tutorial

added support for Ximea cameras (patch by Marian Zajko), fixed GCC warnings in MOG2 background subtractor.