Fix camera AutoFocus does not work correctly on extremely small models (e.g. fly leg)

[adamkewley]

Because the auto-focus algorithm was made tighter (#551) and made to focus the center of the model, rather than {0,0,0} (#550) , but those new constraints made it zoom too tightly on empty models, which is bad UX for new users that won't know that they need to zoom out (#552),