If we have a label image:
[[1, 2],
[3, 4]]
Then the footprint:
[[0, 0, 0],
[0, 1, 1],
[0, 1, 1]]
will not discover the edge (2, 3), even though that edge should be
present when `connectivity=2`.
This commit fixes the bug by using a complete footprint, without the
top/left surfaces zeroed out.
See also:
https://github.com/scikit-image/scikit-image/pull/1826#issuecomment-164597442