Set a sensible viewBox

As today, the viewBox setting is commented out, I do not really know what that means and what could be a sensible value for it, so any help to figured it out is welcome, in any case things seems to work okish even without it.

here relevant code:  https://github.com/tglman/tileline/blob/4f3e08f19cc6d0fece57be7c36e220d14f679177/src/pieces.rs#L22