fixing linewidth animation zoom distortion

When zoomed in, the animated linewidth is very distorted. This PR fixes it, so that linewidth is always relative to the radius.

From one of the demos, demonstrating the issue:
![selection_041](https://f.cloud.github.com/assets/112170/1831294/6ce85b90-7389-11e3-83db-9c8ffa3fc640.png)