title: cv2.fillConvexPoly and cv2.polylines yield different edges?
body: <p>After drawing adjacent filled polygons with cv2.fillConvexPoly, edges between the polygons are drawn using cv2.polylines</p>
  <p><strong>Expected behavior</strong>: The edges nicely fit in between two drawn polygons</p>
  <p><strong>Actual behavior</strong>: Some edges actually lie inside one polygon! (see picture below)</p>
  <p><a href="https://i.stack.imgur.com/O9SGS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O9SGS.png" alt="enter image description here" /></a></p>
  <p><strong>Code to reproduce</strong></p>
  <pre><code># params
  h,w = 20,30
  points_yx = [(5, 1), (7, 22), (16, 13)]
  points_xy = [(x,y) for y,x in points_yx]
  
  # create voronoi partitiion
  rect = (0, 0, w,h)
  subdiv = cv2.Subdiv2D(rect)
  subdiv.insert(points_xy)
  polygons, centers = subdiv.getVoronoiFacetList([])
  polygons_int = [np.round(poly).astype(np.int) for poly in polygons]
  
  # draw voronoi polygons on image
  img = np.zeros((h, w))
  for i, poly in enumerate(polygons_int):
      color = i + 1
      cv2.fillConvexPoly(img, poly, color=color, lineType=cv2.LINE_8)
      x,y = points_xy[i]
      img[y, x] = 0
  
  # draw borders of polygons
  cv2.polylines(img, polygons_int, isClosed=False, color=0, thickness=0,
                lineType=cv2.LINE_8)
  
  # plot
  fig, ax = plt.subplots(1, 1)
  ax.imshow(img, interpolation=None)
  plt.show()
  </code></pre>