Fix #1269 - Transport Helicopter Loading while in the Air

Note that weight is now checked for cargo (this might break things)

OpenRA.Mods.RA/Activities/EnterTransport.cs

@@ -27,10 +27,9 @@ public override Activity Tick(Actor self)
 			if (transport == null || !transport.IsInWorld) return NextActivity;
 
 			var cargo = transport.Trait<Cargo>();
-			if (!cargo.HasSpace(1))
+			if (!cargo.CanLoad(transport, self))
 				return NextActivity;
 
-
 			// Todo: Queue a move order to the transport? need to be
 			// careful about units that can't path to the transport
 			if ((transport.Location - self.Location).LengthSquared > 2)

OpenRA.Mods.RA/Cargo.cs

@@ -87,6 +87,16 @@ bool CanUnload(Actor self)
 			return true;
 		}
 
+		public bool CanLoad(Actor self, Actor a)
+		{
+			if (!HasSpace(GetWeight(a)))
+				return false;
+
+			// Cannot load mid-air
+			var move = self.TraitOrDefault<IMove>();
+			return move == null || move.Altitude == 0;
+		}
+
 		public string CursorForOrder(Actor self, Order order)
 		{
 			if (order.OrderString != "Unload") return null;