Measure afterburner cooldowns on the mission clock (#7895)

* Measure afterburner cooldowns on the mission clock

afterburners_start()/afterburners_stop() timed the +Aburn Cooldown Time delay and
the player's 1.3 s re-engage delay with timer_get_milliseconds(), the real
clock. Under time compression a cooldown therefore lasted longer in mission
time than the table says, and it kept running while the game was paused. The
cooldown was added in 2020 and reused the retail re-engage check's real-clock
"now"; the rest of the file (ABURN_DECAY_TIME, the sound timers) already uses
mission timestamps.

Both delays now use the mission clock. The mission clock does not run outside
missions, so it can still be near zero when the first mission of a session
starts; the "last stopped" and "last started" times therefore start out
invalid instead of 0, so that no delay is in effect before the afterburner has
been used.

Without time compression or pausing, the delays behave as before.

* Drop stale afterburner comment and use fl2i for the cooldown cast

The comment described an earlier startup state rather than the current
behavior. fl2i is the project's float-to-int macro and truncates the same
way the old C-style cast did.

* Remove the remaining afterburner cooldown comment

Timestamps running on mission time is the common case, so it doesn't
need a comment.

code/ship/afterburner.cpp

@@ -29,7 +29,7 @@ static sound_handle Player_afterburner_loop_id; // identifies the looping afterb
 static int		Player_afterburner_loop_delay;	// timestamp used to time the start of the looping afterburner sound
 static int		Player_disengage_timer;
 static float	Player_afterburner_vol;
-static int		Player_afterburner_start_time;
+static TIMESTAMP	Player_afterburner_start_time;
 
 
 // ----------------------------------------------------------
@@ -53,7 +53,7 @@ void afterburner_level_init()
 	Player_disengage_timer = 1;
 	Player_afterburner_vol = AFTERBURNER_DEFAULT_VOL;
 	Player_afterburner_loop_id    = sound_handle::invalid();
-	Player_afterburner_start_time = 0;
+	Player_afterburner_start_time = TIMESTAMP::invalid();
 }
 
 /**
@@ -88,9 +88,7 @@ void afterburners_start(object *objp)
 
 	shipp->flags.set(Ship::Ship_Flags::Attempting_to_afterburn);
 
-	int now = timer_get_milliseconds();
-
-	if (now - shipp->afterburner_last_end_time < (int)(sip->afterburner_cooldown_time * 1000.0f)) {
+	if (shipp->afterburner_last_end_time.isFinite() && timestamp_since(shipp->afterburner_last_end_time) < fl2i(sip->afterburner_cooldown_time * 1000.0f)) {
 
 		if ((objp->flags[Object::Object_Flags::Player_ship]) && (objp == Player_obj))
 			snd_play(gamesnd_get_game_sound(ship_get_sound(objp, GameSounds::ABURN_FAIL)));
@@ -101,7 +99,7 @@ void afterburners_start(object *objp)
 
 	if ( (objp->flags[Object::Object_Flags::Player_ship]) && (objp == Player_obj) ) {
 		
-		if ( (now - Player_afterburner_start_time) < 1300 ) {
+		if ( Player_afterburner_start_time.isFinite() && timestamp_since(Player_afterburner_start_time) < 1300 ) {
 			snd_play( gamesnd_get_game_sound(ship_get_sound(objp, GameSounds::ABURN_FAIL)) );
 			return;
 		}
@@ -143,7 +141,7 @@ void afterburners_start(object *objp)
 	Ship_info[shipp->ship_info_index].animations.getAll(model_get_instance(shipp->model_instance_num), animation::ModelAnimationTriggerType::Afterburner).start(animation::ModelAnimationDirection::FWD);
 
 	if ( objp == Player_obj ) {
-		Player_afterburner_start_time = timer_get_milliseconds();
+		Player_afterburner_start_time = _timestamp();
 		Player_disengage_timer = 1;
 		Player_afterburner_vol = AFTERBURNER_DEFAULT_VOL;
 
@@ -330,7 +328,7 @@ void afterburners_stop(object *objp, int key_released)
 		return;
 	}
 
-	shipp->afterburner_last_end_time = timer_get_milliseconds();
+	shipp->afterburner_last_end_time = _timestamp();
 
 	if (scripting::hooks::OnAfterburnerEnd->isActive()) {
 		scripting::hooks::OnAfterburnerEnd->run(scripting::hooks::ShipSourceConditions{ shipp },

code/ship/ship.cpp

@@ -6970,7 +6970,7 @@ void ship::clear()
 	
 	afterburner_fuel = 0.0f;
 	afterburner_last_engage_fuel = 0.0f;
-	afterburner_last_end_time = 0;
+	afterburner_last_end_time = TIMESTAMP::invalid();
 
 	cmeasure_count = 0;
 	current_cmeasure = -1;

code/ship/ship.h

@@ -756,7 +756,7 @@ public:
 	float	afterburner_fuel;					// amount of afterburner fuel remaining (capacity is stored
 												// as afterburner_fuel_capacity in ship_info).
 	float   afterburner_last_engage_fuel;      // the fuel level when the afterburners were last engaged
-	int   afterburner_last_end_time;         // timestamp when the ship last stopped its afterburner
+	TIMESTAMP afterburner_last_end_time;     // when the ship last stopped its afterburner
 
 	int cmeasure_count;						//	Number of charges of countermeasures this ship can hold.
 	int current_cmeasure;					//	Currently selected countermeasure.