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.

modules/highgui/src/cap.cpp

@@ -489,14 +489,14 @@ VideoCapture::~VideoCapture()
 
 bool VideoCapture::open(const string& filename)
 {
-    if (!isOpened())
+    if (isOpened()) release();
     cap = cvCreateFileCapture(filename.c_str());
     return isOpened();
 }
 
 bool VideoCapture::open(int device)
 {
-    if (!isOpened())
+    if (isOpened()) release();
     cap = cvCreateCameraCapture(device);
     return isOpened();
 }