Add helper methods to locate actors that can be reached via a path.

Previously, the ClosestTo and PositionClosestTo existed to perform a simple distance based check to choose the closest location from a choice of locations to a single other location. For some functions this is sufficient, but for many functions we want to then move between the locations. If the location selected is in fact unreachable (e.g. on another island) then we would not want to consider it.

We now introduce ClosestToIgnoringPath for checks where we don't care about a path existing, e.g. weapons hitting nearby targets. When we do care about paths, we introduce ClosestToWithPathFrom and ClosestToWithPathTo which will check that a path exists. The PathFrom check will make sure one of the actors from the list can make it to the single target location. The PathTo check will make sure the single actor can make it to one of the target locations. This difference allows us to specify which actor will be doing the moving. This is important as a path might exists for one actor, but not another. Consider two islands with a hovercraft on one and a tank on the other. The hovercraft can path to the tank, but the tank cannot path to the hovercraft.

We also introduce WithPathFrom and WithPathTo. These will perform filtering by checking for valid paths, but won't select the closest location.

By employing the new methods that filter for paths, we fix various behaviour that would cause actors to get confused. Imagine an islands map, by checking for paths we ensure logic will locate reachable locations on the island, rather than considering a location on a nearby island that is physically closer but unreachable. This fixes AI squad automation, and other automated behaviours such as rearming.

OpenRA.Game/WorldUtils.cs

@@ -19,19 +19,51 @@ namespace OpenRA
 {
 	public static class WorldUtils
 	{
-		public static Actor ClosestTo(this IEnumerable<Actor> actors, Actor a)
+		/// <summary>
+		/// From the given <paramref name="actors"/>, select the one nearest the given <paramref name="actor"/> by
+		/// comparing their <see cref="Actor.CenterPosition"/>. No check is done to see if a path exists.
+		/// </summary>
+		public static Actor ClosestToIgnoringPath(this IEnumerable<Actor> actors, Actor actor)
 		{
-			return actors.ClosestTo(a.CenterPosition);
+			return actors.ClosestToIgnoringPath(actor.CenterPosition);
 		}
 
-		public static Actor ClosestTo(this IEnumerable<Actor> actors, WPos pos)
+		/// <summary>
+		/// From the given <paramref name="actors"/>, select the one nearest the given <paramref name="position"/> by
+		/// comparing the <see cref="Actor.CenterPosition"/>. No check is done to see if a path exists.
+		/// </summary>
+		public static Actor ClosestToIgnoringPath(this IEnumerable<Actor> actors, WPos position)
 		{
-			return actors.MinByOrDefault(a => (a.CenterPosition - pos).LengthSquared);
+			return actors.MinByOrDefault(a => (a.CenterPosition - position).LengthSquared);
 		}
 
-		public static WPos PositionClosestTo(this IEnumerable<WPos> positions, WPos pos)
+		/// <summary>
+		/// From the given <paramref name="items"/> that can be projected to <see cref="Actor"/>,
+		/// select the one nearest the given <paramref name="actor"/> by
+		/// comparing their <see cref="Actor.CenterPosition"/>. No check is done to see if a path exists.
+		/// </summary>
+		public static T ClosestToIgnoringPath<T>(IEnumerable<T> items, Func<T, Actor> selector, Actor actor)
 		{
-			return positions.MinByOrDefault(p => (p - pos).LengthSquared);
+			return ClosestToIgnoringPath(items, selector, actor.CenterPosition);
+		}
+
+		/// <summary>
+		/// From the given <paramref name="items"/> that can be projected to <see cref="Actor"/>,
+		/// select the one nearest the given <paramref name="position"/> by
+		/// comparing the <see cref="Actor.CenterPosition"/>. No check is done to see if a path exists.
+		/// </summary>
+		public static T ClosestToIgnoringPath<T>(IEnumerable<T> items, Func<T, Actor> selector, WPos position)
+		{
+			return items.MinByOrDefault(x => (selector(x).CenterPosition - position).LengthSquared);
+		}
+
+		/// <summary>
+		/// From the given <paramref name="positions"/>, select the one nearest the given <paramref name="position"/>.
+		/// No check is done to see if a path exists, as an actor is required for that.
+		/// </summary>
+		public static WPos ClosestToIgnoringPath(this IEnumerable<WPos> positions, WPos position)
+		{
+			return positions.MinByOrDefault(p => (p - position).LengthSquared);
 		}
 
 		public static IEnumerable<Actor> FindActorsInCircle(this World world, WPos origin, WDist r)