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; }
 	}
 

OpenRA.Game/ActorMap.cs

@@ -64,6 +64,16 @@ namespace OpenRA
 				SubCell.BottomLeft, SubCell.BottomRight }.Any(b => !AnyUnitsAt(a,b));
 		}
 
+		public SubCell? FreeSubCell(CPos a)
+		{
+			if (!HasFreeSubCell(a))
+				return null;
+
+			return new[]{ SubCell.TopLeft, SubCell.TopRight, SubCell.Center,
+				SubCell.BottomLeft, SubCell.BottomRight }.First(b => !AnyUnitsAt(a,b));
+		}
+
+
 		public bool AnyUnitsAt(CPos a)
 		{
 			return influence[ a.X, a.Y ] != null;