A dependency cycle occurs currently because the camera's properties
depend on both the armature and itself, through the lens property.
As far as I can tell this cycle does not cause any lag, but it's
better to fix it anyway.
This commit works around this cycle by introducing an intermediate
lens property on the rig itself, and this is used to drive the actual
camera lens.
Reported and fixed by Beorn Leonard.
Wayne Dixon
Damien Picard