Vertex2D objects do not render in webviewer

On commit 505a0478ba44d46b6730e122759412a67961bf10 , I get a black screen when I run:
```python
vtx_2d = cortex.Vertex2D(cortex.Vertex.random('S1'), cortex.Vertex.random('S1'))
cortex.webshow(vtx_2d, open_browser=False, autoclose=True)
```

After bisecting, I believe this is the problem commit (PR #679): 0fa99b3b0a123578ebbddfe01e124e3376b96fbe