Fix some incorrect logic in PathGraph.GetConnections.PathGraph

Firstly, when dealing with maps with height discontinuities, the neighbouring cells we need to search are more that the set we need to search on flat maps. We ensure that as we traverse a map with varying height, we now consider cells "behind" us that may have become accessible due to a height change.

Secondly, when considering connections available via Custom Movement Layers, make sure the target cell on the new layer is actually enterable. Previously this cell would be reported as a valid connection, even if it wasn't actually possible to enter the cell as it was blocked. We also apply the same optimization of ignoring already closed cells.

OpenRA.Game/Exts.cs

@@ -375,6 +375,11 @@ namespace OpenRA
 			return ts.Concat(moreTs);
 		}
 
+		public static IEnumerable<T> Exclude<T>(this IEnumerable<T> ts, params T[] exclusions)
+		{
+			return ts.Except(exclusions);
+		}
+
 		public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source)
 		{
 			return new HashSet<T>(source);