3D viewer: replace hidden spheres in SVG export with short lines
3D viewer: replace hidden spheres in SVG export with short lines

For each replacement line the diameter of the original sphere is
computed and used as the stroke-width in screen space. These lines are
added temporarily to the scene and removed after the export.

Closes #821.

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID