Merge HeliAttack into FlyAttack

And polish CanHover FlyAttack behavior:

- Get rid of direct TickFacing usage
- Fix that the CanHover facing/altitude update would override
  TakeOff child of Fly
- Streamline the queueing of child activities
- Get rid of a direct FlyTick in favor of relying on Fly activity
- Pull queueing of TakeOff out of the if-else

OpenRA.Mods.Common/Activities/Air/FlyAttack.cs

@@ -150,14 +150,29 @@ public override Activity Tick(Actor self)
 				return this;
 			}
 
-			if (self.World.Map.DistanceAboveTerrain(self.CenterPosition).Length < aircraft.Info.MinAirborneAltitude)
+			var delta = attackAircraft.GetTargetPosition(pos, target) - pos;
+			var desiredFacing = delta.HorizontalLengthSquared != 0 ? delta.Yaw.Facing : aircraft.Facing;
+			var isAirborne = self.World.Map.DistanceAboveTerrain(pos).Length >= aircraft.Info.MinAirborneAltitude;
+
+			if (!isAirborne)
 				QueueChild(self, new TakeOff(self), true);
 
-			if (attackAircraft != null && target.IsInRange(self.CenterPosition, attackAircraft.GetMinimumRange()))
-				QueueChild(self, new FlyTimed(ticksUntilTurn, self), true);
+			if (!aircraft.Info.CanHover)
+			{
+				if (target.IsInRange(pos, attackAircraft.GetMinimumRange()))
+					QueueChild(self, new FlyTimed(ticksUntilTurn, self), true);
 
-			QueueChild(self, new Fly(self, target, checkTarget.CenterPosition, Color.Red), true);
-			QueueChild(self, new FlyTimed(ticksUntilTurn, self));
+				QueueChild(self, new Fly(self, target, target.CenterPosition, Color.Red), true);
+				QueueChild(self, new FlyTimed(ticksUntilTurn, self));
+			}
+			else
+			{
+				var minimumRange = attackAircraft.GetMinimumRangeVersusTarget(target);
+				if (!target.IsInRange(pos, lastVisibleMaximumRange) || target.IsInRange(pos, minimumRange))
+					QueueChild(self, new Fly(self, target, minimumRange, lastVisibleMaximumRange, target.CenterPosition, Color.Red), true);
+				else if (isAirborne) // Don't use 'else' to avoid conflict with TakeOff
+					Fly.VerticalTakeOffOrLandTick(self, aircraft, desiredFacing, aircraft.Info.CruiseAltitude);
+			}
 
 			return this;
 		}

OpenRA.Mods.Common/Traits/Air/AttackAircraft.cs

@@ -37,9 +37,6 @@ public AttackAircraft(Actor self, AttackAircraftInfo info)
 
 		public override Activity GetAttackActivity(Actor self, Target newTarget, bool allowMove, bool forceAttack)
 		{
-			if (aircraftInfo.CanHover)
-				return new HeliAttack(self, newTarget, forceAttack);
-
 			return new FlyAttack(self, newTarget, forceAttack);
 		}