title: imshow() with desired framerate with opencv
body: <p>Is there any workaround how to use <code>cv2.imshow()</code> with a specific framerate? Im capturing the video via VideoCapture and doing some easy postprocessing on them (both in a separeted thread, so it loads all frames in Queue and the main thread isn't slowed by the computation). I tryed to fix the framerate by calculating the time used for "reading" the image from the queue and then substract that value from number of miliseconds avalible for one frame:
  if I have as input video with 50FPS and i want to playback it in real-time i do 1000/50 => 20ms per frame. 
  And then wait that time using cv2.WaitKey()
  But still I get some laggy output. Which is slower then the source video</p>