Fix: thread safety issue during exiting the game (#9380)

_exit_game is read by the draw-thread to know when to exit, but
most of the time written by the game-thread.

(cherry picked from commit c12a152ec9)

src/gfx.cpp

@@ -48,7 +48,7 @@ bool _right_button_clicked; ///< Is right mouse button clicked?
 DrawPixelInfo _screen;
 bool _screen_disable_anim = false;   ///< Disable palette animation (important for 32bpp-anim blitter during giant screenshot)
 bool _check_special_modes;
-bool _exit_game;
+std::atomic<bool> _exit_game;
 GameMode _game_mode;
 SwitchMode _switch_mode;  ///< The next mainloop command.
 PauseMode _pause_mode;

src/openttd.h

@@ -10,6 +10,7 @@
 #ifndef OPENTTD_H
 #define OPENTTD_H
 
+#include <atomic>
 #include "core/enum_type.hpp"
 
 /** Mode which defines the state of the game. */
@@ -57,7 +58,7 @@ enum ExtraDisplayOptions {
 extern GameMode _game_mode;
 extern SwitchMode _switch_mode;
 extern bool _check_special_modes;
-extern bool _exit_game;
+extern std::atomic<bool> _exit_game;
 extern bool _save_config;
 
 /** Modes of pausing we've got */

src/video/null_v.cpp

@@ -15,12 +15,14 @@
 #include "../thread.h"
 #include "null_v.h"
 
+#include <atomic>
+
 #include "../safeguards.h"
 
 /** Factory for the null video driver. */
 static FVideoDriver_Null iFVideoDriver_Null;
 
-extern bool _exit_game;
+extern std::atomic<bool> _exit_game;
 
 const char *VideoDriver_Null::Start(const StringList &parm)
 {