sim: drop HFRs during prespawn

This fixes the brief flash of a big HFR list and also works around an
as-yet unresolved bug where aircraft would not be in the STARSComputer HFR
list but would be in the Sim DepartureState, which in turn prevented
departures from launching since the Sim thought there was an unreleased one
in front.

Author: mmp

pkg/sim/spawn.go

@@ -508,7 +508,9 @@ func (s *Sim) spawnDepartures() {
 			// passed since the last one.
 			if now.After(depState.NextIFRSpawn) {
 				if ac, err := s.makeNewIFRDeparture(airport, runway); ac != nil && err == nil {
-					if !s.prespawnUncontrolledOnly || !s.isControlled(ac, true) { // keep virtual-controller-only ones
+					dropUncontrolled := s.prespawnUncontrolledOnly && s.isControlled(ac, true)
+					dropHFR := s.prespawn && ac.HoldForRelease
+					if !dropUncontrolled && !dropHFR {
 						s.addDepartureToPool(ac, runway)
 						r := scaleRate(depState.IFRSpawnRate, s.State.LaunchConfig.DepartureRateScale)
 						depState.NextIFRSpawn = now.Add(randomWait(r, false))