fixed warnings in gpu modules from VS 2012
Fixed VideoCapture::open() does not release previous capture sources (Bug #3150).

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.
Merge pull request #1082 from jet47:gpu-warnings-fix
Merge pull request #1104 from nikman-ru:cap_reuse