Existing shadow caster culling using the BVH takes no account of the camera. This PR adds the highly encapsulated class VisualServerLightCuller which can cut down the casters in the shadow volume to only those which can cast shadows on the camera frustum.
This is used to:
* More accurately defer dirty updates to shadows when the shadow volume does not intersect the camera frustum.
* Tighter cull shadow casters to the view frustum.
Lights dirty state is now automatically managed:
* Continuous (tighter caster culling)
* Static (all casters are rendered)
