re/segment join_polygons: fix rare case of adjacent rings

Author: Robert Sachunsky

ocrd_cis/ocropy/segment.py

@@ -908,7 +908,7 @@ def join_polygons(polygons, loc='', scale=20):
     dists = np.eye(npoly, dtype=float)
     for i, j in pairs:
         dist = polygons[i].distance(polygons[j])
-        if dist == 0:
+        if dist < 1e-5:
             dist = 1e-5 # if pair merely touches, we still need to get an edge
         dists[i, j] = dist
         dists[j, i] = dist