text: Title: TypeError when trying to output PNG frames
  Body: I'm getting this error if I add the --png argument:
  
  `xception ignored in thread started by: <function clear_buffer at 0x00000158A426F310>
  Traceback (most recent call last):
    File "inference_video_parallel.py", line 83, in clear_buffer
      cv2.imwrite('output/{:0>7d}.png'.format(cnt), i)
  TypeError: unsupported format string passed to numpy.ndarray.__format__`