Improve support for picking a valid actor spawn cell.

OpenRA.Game/ActorInitializer.cs

@@ -75,6 +75,7 @@ namespace OpenRA
 		[FieldFromYamlKey] public readonly int value = 0;
 		public SubCellInit() { }
 		public SubCellInit(int init) { value = init; }
+		public SubCellInit(SubCell init) { value = (int)init; }
 		public SubCell Value(World world) { return (SubCell)value; }
 	}