Save volume rendering as STL file

[lassoan (Andras Lasso)]

Volume rendering is just a display technique - to get an STL file you need to segment the volume using Segment Editor module.

• Tutorial: https://www.slicer.org/wiki/Documentation/4.6/Training#Segmentation_for_3D_printing
• Reference: http://slicer.readthedocs.io/en/latest/user_guide/module_segmenteditor.html