Expose the current camera viewport when tracking

[ianthetechie]

I was discussing a scenario today with @Patrick-Kladek and he suggested that it would be useful to include the visible bounding box as part of this state.

I'm not quite sure how well this would work via two-way binding but worth a discussion. We would need to be careful to not cause excessive CPU usage / view recomputation via extra updates.

An alternate option might be to propagate the delegate updates somehow.