Improve line rendering of GeometryRenderer

The GeometryRenderer is the bread-and-butter processor for rendering geometry. However, its implementation of rendering lines is still a hack:

• setting wireframe flag, sometimes it uses the wireframe color.
• the shading is done on non-existent normal vectors
• linewidth is not set

Fix and clean the implementation and perhaps even merge with the fiber renderer (supporting on-the-fly creation of tubes or fake-tubes)