center avatar when calculating aabb

before:

![screen shot 2013-07-23 at 8 41 00 pm](https://f.cloud.github.com/assets/39759/846723/c6dc2b64-f421-11e2-89e5-310e1e630d3b.png)

after:

![screen shot 2013-07-23 at 10 27 27 pm](https://f.cloud.github.com/assets/39759/846724/cb6c72c4-f421-11e2-9cfb-4c91c2f91022.png)