Codechange: introduce a framework for all our timers

IntervalTimer and TimeoutTimer use RAII, and can be used to replace
all the time-based timeouts, lag-detection, "execute every N" we
have.
As it uses RAII, you can safely use it as static variable, class
member, temporary variable, etc. As soon as it goes out-of-scope,
it will be safely removed.
This allows for much easier to read code when it comes to intervals.

src/timer/timer_game_calendar.cpp

@@ -0,0 +1,89 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @file timer_game_calendar.cpp
+ * This file implements the timer logic for the game-calendar-timer.
+ */
+
+#include "stdafx.h"
+#include "date_func.h"
+#include "openttd.h"
+#include "timer.h"
+#include "timer_game_calendar.h"
+#include "vehicle_base.h"
+#include "linkgraph/linkgraph.h"
+
+#include "safeguards.h"
+
+template<>
+void IntervalTimer<TimerGameCalendar>::Elapsed(TimerGameCalendar::TElapsed trigger)
+{
+	if (trigger == this->period.trigger) {
+		this->callback(1);
+	}
+}
+
+template<>
+void TimeoutTimer<TimerGameCalendar>::Elapsed(TimerGameCalendar::TElapsed trigger)
+{
+	if (this->fired) return;
+
+	if (trigger == this->period.trigger) {
+		this->callback();
+		this->fired = true;
+	}
+}
+
+template<>
+void TimerManager<TimerGameCalendar>::Elapsed(TimerGameCalendar::TElapsed delta)
+{
+	assert(delta == 1);
+
+	_tick_counter++;
+
+	if (_game_mode == GM_MENU) return;
+
+	_date_fract++;
+	if (_date_fract < DAY_TICKS) return;
+	_date_fract = 0;
+
+	/* increase day counter */
+	_date++;
+
+	YearMonthDay ymd;
+	ConvertDateToYMD(_date, &ymd);
+
+	/* check if we entered a new month? */
+	bool new_month = ymd.month != _cur_month;
+
+	/* check if we entered a new year? */
+	bool new_year = ymd.year != _cur_year;
+
+	/* update internal variables before calling the daily/monthly/yearly loops */
+	_cur_month = ymd.month;
+	_cur_year  = ymd.year;
+
+	/* Make a temporary copy of the timers, as a timer's callback might add/remove other timers. */
+	auto timers = TimerManager<TimerGameCalendar>::GetTimers();
+
+	for (auto timer : timers) {
+		timer->Elapsed(TimerGameCalendar::DAY);
+	}
+
+	if (new_month) {
+		for (auto timer : timers) {
+			timer->Elapsed(TimerGameCalendar::MONTH);
+		}
+	}
+
+	if (new_year) {
+		for (auto timer : timers) {
+			timer->Elapsed(TimerGameCalendar::YEAR);
+		}
+	}
+}