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.

src/gfx.cpp

@@ -41,7 +41,7 @@ bool _right_button_down;    ///< Is right mouse button pressed?
 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 _exit_game;
+std::atomic<bool> _exit_game;
 GameMode _game_mode;
 SwitchMode _switch_mode;  ///< The next mainloop command.
 PauseMode _pause_mode;