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;

OpenRA.Mods.RA/Move/Mobile.cs

@@ -109,6 +109,11 @@ namespace OpenRA.Mods.RA.Move
 			return true;
 		}
 
+		public bool CanEnterCell(World world, CPos cell)
+		{
+			return CanEnterCell(world, null, cell, null, true, true);
+		}
+
 		public bool CanEnterCell(World world, Actor self, CPos cell, Actor ignoreActor, bool checkTransientActors, bool blockedByMovers)
 		{
 			if (MovementCostForCell(world, cell) == int.MaxValue)
@@ -120,13 +125,13 @@ namespace OpenRA.Mods.RA.Move
 			var blockingActors = world.ActorMap.GetUnitsAt(cell)
 				.Where(x => x != ignoreActor)
 				// Neutral/enemy units are blockers. Allied units that are moving are not blockers.
-				.Where(x => blockedByMovers || ((self.Owner.Stances[x.Owner] != Stance.Ally) || !IsMovingInMyDirection(self, x)))
+				.Where(x => blockedByMovers || (self == null || self.Owner.Stances[x.Owner] != Stance.Ally || !IsMovingInMyDirection(self, x)))
 				.ToList();
 
 			if (checkTransientActors && blockingActors.Count > 0)
 			{
 				// Non-sharable unit can enter a cell with shareable units only if it can crush all of them
-				if (Crushes == null)
+				if (self == null || Crushes == null)
 					return false;
 
 				if (blockingActors.Any(a => !(a.HasTrait<ICrushable>() &&