Add: adhere the autosave_on_exit setting for Null videodriver (#9343)

This is especially useful for automated-testing, to make a save
when the game quits while using "-vnull:ticks=N".

src/video/null_v.cpp

@@ -10,6 +10,7 @@
 #include "../stdafx.h"
 #include "../gfx_func.h"
 #include "../blitter/factory.hpp"
+#include "../saveload/saveload.h"
 #include "../window_func.h"
 #include "null_v.h"
 
@@ -52,6 +53,12 @@ void VideoDriver_Null::MainLoop()
 		::InputLoop();
 		::UpdateWindows();
 	}
+
+	/* If requested, make a save just before exit. The normal exit-flow is
+	 * not triggered from this driver, so we have to do this manually. */
+	if (_settings_client.gui.autosave_on_exit) {
+		DoExitSave();
+	}
 }
 
 bool VideoDriver_Null::ChangeResolution(int w, int h) { return false; }