TreeView misses some edges when FisheyeTreeFilter depthLevel = 0

[markhm]

As title states. The TreeView (demo) misses some edges when depthLevel = 0.

I've described steps to reproduce, images showing the problem and sample code here:

https://github.com/markhm/prefuse_demos/

[alex-rind]

Experimental bugfix to prefuse's FisheyeTreeFilter in a feature branch linked above.