Fix slow shutdowns on error

SDL_threadID was being compared to int and woudl thus always fail,
resulting in the application thinking that another thread was trying to
shutdown.

Source/appfat.cpp

@@ -14,7 +14,7 @@ char sz_error_buf[256];
 /** Set to true when a fatal error is encountered and the application should shut down. */
 BOOL terminating;
 /** Thread id of the last callee to FreeDlg(). */
-int cleanup_thread_id;
+SDL_threadID cleanup_thread_id;
 
 /**
  * @brief Terminates the game and displays an error message box.

Source/appfat.h

@@ -14,7 +14,6 @@ extern "C" {
 
 extern char sz_error_buf[256];
 extern BOOL terminating;
-extern int cleanup_thread_id;
 
 void TriggerBreak();
 char *GetErrorStr(DWORD error_code);