Display surface in3d image viewer

New option to display digital surface instead all the voxels (more efficient).
And also a possibility to disting Connected components with colors...
![capture decran 2013-07-02 a 13 41 49](https://f.cloud.github.com/assets/772865/736155/6d69e8b2-e30c-11e2-85a9-8eda81c1d5e2.png)