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.

[NSRunLoop currentRunLoop] is not the same as in the main thread.
see
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/F
oundation/Classes/nsrunloop_Class/Reference/Reference.html
"If no input sources or timers are attached to the run loop, this
method exits immediately"

The old code caused grabFrame() to return immediately if the capture
was not done in the main thread.
This code works correctly, even in secondary threads, provided that all
capture functions are called in the *same* thread

Calib3d badarg tests fixed. Default exception handling enabled;
Highgui Video I/O tests fixed on ARM;
Code cleanup.

Windows RT sample cleanup.
Video I/O code cleanup.

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

Video i/o tests enabled for media foundation;
Negative stride support added to VideoCapture;
Error handling improved, dead lock in case of playback error fixed;
Some code refacotring done.

I* + SafeRelease -> ComPtr.

Camera-based VideoCapture updated to fit changes in ImageGrabber from prev
commit

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

FourCC parameter handlig added;
Smart pointers instead SafeRelease call;
Windows RT support (vertical mirroring).

Code formating fixed;
GrabFrame method implemented correclty.

I don't know why it didn't work for the original author, but it definitely
works now.

Updated iOS camera. Added rotation flag. Added functions to lock/unlock focus, white balance and exposure.

Also, removing explicit include path configuration,
since QT_USE_FILE takes care of that.

Based on pull request #685

Set ptr to NULL, so this method can be called repeatedly. This fixes a crash after unplugging web cam and trying to re-scan the cameras.