title: error with cv2.destroyAllWindws() on ubuntu 14.04
body: <p>I'm programing in python and opencv on Ubuntu but the cv2.destoyAllWindows() doesn't seem to work,as in this example:</p>
  
  <pre><code>import numpy as np
  import cv2
  
  cap = cv2.VideoCapture(0)
  
  while(True):
  # Capture frame-by-frame
      ret, frame = cap.read()
  
  # Our operations on the frame come here
      gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  
  # Display the resulting frame
      cv2.imshow('frame',gray)
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  
  # When everything done, release the capture
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>when i press 'q' the program end but the window remain on the screen how ever on windows i didn't face such thing
  any help I will be thankful</p>