Only use largest blocker instead of largest actor for projectile blocker overscan

Before this, we unconditionally used the largest OuterRadius of all actors inside a mod for overscanning of blockable projectiles.
However, in many mods the only blockable actors are 1-cell walls, and even if there are gates like in TS, they usually aren't the largest actors in terms of hit-shape.

So this measure should save a bit of performance by reducing the overscan radius of blockable projectiles, especially in mods where walls are the only blocking actors.

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -259,6 +259,7 @@ namespace OpenRA.Traits
 		IEnumerable<Actor> ActorsInBox(WPos a, WPos b);
 
 		WDist LargestActorRadius { get; }
+		WDist LargestBlockingActorRadius { get; }
 	}
 
 	public interface IRenderModifier