Codechange: use std::chrono to track time in modal windows

Adding to _realtime_ticks in a random place is a bit of a hack,
and by using modern C++, we can avoid this hack.

src/genworld_gui.cpp

@@ -1188,7 +1188,7 @@ struct GenWorldStatus {
 	StringID cls;
 	uint current;
 	uint total;
-	int timer;
+	std::chrono::steady_clock::time_point timer;
 };
 
 static GenWorldStatus _gws;
@@ -1294,7 +1294,7 @@ void PrepareGenerateWorldProgress()
 	_gws.current = 0;
 	_gws.total   = 0;
 	_gws.percent = 0;
-	_gws.timer   = 0; // Forces to paint the progress window immediately
+	_gws.timer   = std::chrono::steady_clock::now() - std::chrono::milliseconds(MODAL_PROGRESS_REDRAW_TIMEOUT * 2); // Ensure we draw on first update
 }
 
 /**
@@ -1329,7 +1329,7 @@ static void _SetGeneratingWorldProgress(GenWorldProgress cls, uint progress, uin
 	}
 
 	/* Don't update the screen too often. So update it once in every once in a while... */
-	if (!_network_dedicated && _gws.timer != 0 && _realtime_tick - _gws.timer < MODAL_PROGRESS_REDRAW_TIMEOUT) return;
+	if (!_network_dedicated && std::chrono::steady_clock::now() - _gws.timer < std::chrono::milliseconds(MODAL_PROGRESS_REDRAW_TIMEOUT)) return;
 
 	/* Percentage is about the number of completed tasks, so 'current - 1' */
 	_gws.percent = percent_table[cls] + (percent_table[cls + 1] - percent_table[cls]) * (_gws.current == 0 ? 0 : _gws.current - 1) / _gws.total;
@@ -1365,7 +1365,7 @@ static void _SetGeneratingWorldProgress(GenWorldProgress cls, uint progress, uin
 	_modal_progress_work_mutex.lock();
 	_modal_progress_paint_mutex.unlock();
 
-	_gws.timer = _realtime_tick;
+	_gws.timer = std::chrono::steady_clock::now();
 }
 
 /**

src/gfx.cpp

@@ -1470,9 +1470,8 @@ void DrawDirtyBlocks()
 		_modal_progress_paint_mutex.unlock();
 		_modal_progress_work_mutex.unlock();
 
-		/* Wait a while and update _realtime_tick so we are given the rights */
+		/* Wait a while and hope the modal gives us a bit of time to draw the GUI. */
 		if (!IsFirstModalProgressLoop()) CSleep(MODAL_PROGRESS_REDRAW_TIMEOUT);
-		_realtime_tick += MODAL_PROGRESS_REDRAW_TIMEOUT;
 
 		/* Modal progress thread may need blitter access while we are waiting for it. */
 		VideoDriver::GetInstance()->ReleaseBlitterLock();