Airlift support.

UI isn't finished. Bulk transfers where the player doesn't care what
aircraft get used work (though they're chosen with no thought at all),
but being able to plan your own airlift flight isn't here yet.

Cargo planes are not implemented yet.

No way to view the cargo of a flight (will come with the cargo flight
planning UI).

The airlift flight/package creation should probably be moved out of the
UI and into the game code.

AI doesn't use these yet.

https://github.com/Khopa/dcs_liberation/issues/825

gen/flights/waypointbuilder.py

@@ -16,10 +16,9 @@ from dcs.mapping import Point
 from dcs.unit import Unit
 from dcs.unitgroup import Group, VehicleGroup
 
-from game.transfers import Convoy
-
 if TYPE_CHECKING:
     from game import Game
+    from game.transfers import Convoy
 
 from game.theater import (
     ControlPoint,
@@ -444,24 +443,69 @@ class WaypointBuilder:
         return ingress, waypoint, egress
 
     @staticmethod
-    def nav(position: Point, altitude: Distance) -> FlightWaypoint:
+    def pickup(control_point: ControlPoint) -> FlightWaypoint:
+        """Creates a cargo pickup waypoint.
+
+        Args:
+            control_point: Pick up location.
+        """
+        waypoint = FlightWaypoint(
+            FlightWaypointType.PICKUP,
+            control_point.position.x,
+            control_point.position.y,
+            meters(0),
+        )
+        waypoint.alt_type = "RADIO"
+        waypoint.name = "PICKUP"
+        waypoint.description = f"Pick up cargo from {control_point}"
+        waypoint.pretty_name = "Pick up location"
+        return waypoint
+
+    @staticmethod
+    def drop_off(control_point: ControlPoint) -> FlightWaypoint:
+        """Creates a cargo drop-off waypoint.
+
+        Args:
+            control_point: Drop-off location.
+        """
+        waypoint = FlightWaypoint(
+            FlightWaypointType.PICKUP,
+            control_point.position.x,
+            control_point.position.y,
+            meters(0),
+        )
+        waypoint.alt_type = "RADIO"
+        waypoint.name = "DROP OFF"
+        waypoint.description = f"Drop off cargo at {control_point}"
+        waypoint.pretty_name = "Drop off location"
+        return waypoint
+
+    @staticmethod
+    def nav(
+        position: Point, altitude: Distance, altitude_is_agl: bool = False
+    ) -> FlightWaypoint:
         """Creates a navigation point.
 
         Args:
             position: Position of the waypoint.
             altitude: Altitude of the waypoint.
+            altitude_is_agl: True for altitude is AGL. False if altitude is MSL.
         """
         waypoint = FlightWaypoint(
             FlightWaypointType.NAV, position.x, position.y, altitude
         )
+        if altitude_is_agl:
+            waypoint.alt_type = "RADIO"
         waypoint.name = "NAV"
         waypoint.description = "NAV"
         waypoint.pretty_name = "Nav"
         return waypoint
 
-    def nav_path(self, a: Point, b: Point, altitude: Distance) -> List[FlightWaypoint]:
+    def nav_path(
+        self, a: Point, b: Point, altitude: Distance, altitude_is_agl: bool = False
+    ) -> List[FlightWaypoint]:
         path = self.clean_nav_points(self.navmesh.shortest_path(a, b))
-        return [self.nav(self.perturb(p), altitude) for p in path]
+        return [self.nav(self.perturb(p), altitude, altitude_is_agl) for p in path]
 
     def clean_nav_points(self, points: Iterable[Point]) -> Iterator[Point]:
         # Examine a sliding window of three waypoints. `current` is the waypoint