Fix landing altitude checks.

Both GetPosition and LandPosition were trying to solve the problem
of dynamic altitude offsets for carryalls with different-sized cargo.

However, because they both apply the offsets, when the two are used
together the two sets of offsets cancel to 0, producing incorrect behaviour.

GetPosition is removed in favor of using CenterPosition with the dynamic
LandAltitude. Fly.Tick and Carryall ignore the z coordinate so their
behaviour is unchanged. Land and Aircraft.AtLandAltitude now return
the correct result.

OpenRA.Mods.Common/Traits/Carryall.cs

@@ -309,8 +309,7 @@ namespace OpenRA.Mods.Common.Traits
 			if (IsTraitDisabled)
 				return false;
 
-			var targetCell = self.World.Map.CellContaining(aircraft.GetPosition());
-			return Carryable != null && aircraft.CanLand(targetCell, blockedByMobile: false);
+			return Carryable != null && aircraft.CanLand(self.Location, blockedByMobile: false);
 		}
 
 		IEnumerable<IOrderTargeter> IIssueOrder.Orders