Replace day length scaled tick macros with global variables.

This is to avoid needing to expensively re-calculate them all the time.
Instead update them by incrementing.

src/departures_gui.cpp

@@ -566,7 +566,7 @@ void DeparturesWindow<Twaypoint>::DrawDeparturesListItems(const Rect &r) const
 	uint departure = 0;
 	uint arrival = 0;
 
-	DateTicksScaled now_date = CURRENT_SCALED_TICKS;
+	DateTicksScaled now_date = _scaled_date_ticks;
 	DateTicksScaled max_date = now_date + (_settings_client.gui.max_departure_time * DAY_TICKS * _settings_game.economy.day_length_factor);
 
 	/* Draw each departure. */