Call Button_Quit and Button_Page with correct enum

These functions are called explicitly in a few places and need to be
handed the corresponding enum value to correctly get the button value.
Since they were previously called with -1, they would cause segfaults.
These seemed to happen when restoring backups or loading images to both
pages.

src/engine.c

@@ -730,7 +730,7 @@ void Main_handler(void)
       if (Quit_is_required)
       {
         Quit_is_required=0;
-        Button_Quit(-1);
+        Button_Quit(BUTTON_QUIT);
       }
       
       if (Pan_shortcut_pressed && Current_operation!=OPERATION_PAN_VIEW)