Fix shapes not being drawn with VTK6

Fixes #769

[Test project](https://gist.github.com/VictorLamoine/f0f59e660daa458d1ea5)

![Shapes in VTK6](https://cloud.githubusercontent.com/assets/5566160/4073438/96490982-2e94-11e4-9f12-447043ba059e.png)