VisibilityType.CenterPosition reveals the actor
only if its CenterPosition is inside a cell that
is visible.
VisibilityType.Footprint reveals the actor if any
of its footprint cells are visible (this includes
Mobile actors moving between two cells).
- Don't call string.Format in Actor.ToString since it gets called often, instead prefer a simple concat.
- In HiddenUnderFog.IsVisible, avoid a needless level of lambda indirection.
