Fix depth change and distance map computation.

This commit implements the fixes proposed by James Stout in pcl-dev
mailing list (http://www.pcl-developers.org/Bugs-in-IntegralImageNormalEstimation-distance-map-computation-td5708055.html).

Specifically:
- In depth change map computation the diagonal neighbors are now
  considered.
- In distance map computation Chessboard distance is used instead of an
  approximation of Euclidean distance.
- Use distance map values as smoothing window radius (instead of width).
Add minimum normal smoothing size parameter.

This commit implements the change to IntergalImageNormalEstimation
proposed in pcl-dev mailing list (http://www.pcl-developers.org/Bugs-in-IntegralImageNormalEstimation-distance-map-computation-td5708055.html).