Fix issue with resizing the zoom box width - follow-up of #2816

This fixes an issue with the automatic resizing of the zoom box width.
Currently, if the viewer is small enough that the zoom box isn't visible on load, the zoom box resizing doesn't work. This means that if the viewer is then enlarged, it will look like this:
![zoom-box-issue](https://f.cloud.github.com/assets/2692120/508060/32e8285c-bd7d-11e2-9fbf-37eb2bc35868.png)

This PR fixes this, so that the zoom box will only be resized on load when it's actually visible.