Fix: on startup, NewGRF scan could case race-condition (#9382)

Creating a thread was not thread-safe. The irony.

The video-driver has a function GameLoopPause() which first checks
if the thread is the game-thread or not. For this it needs access
to this->game_thread. This variable is set in StartNewThread().

However, due to timing, it is well possible GameLoopPause() is
called from the thread well before this->game_thread is assigned.

And so we have a race-condition!

Simply solve this by preventing a thread to start till we are
done with our bookkeeping.

src/thread.h

@@ -14,6 +14,7 @@
 #include "crashlog.h"
 #include <system_error>
 #include <thread>
+#include <mutex>
 
 /**
  * Sleep on the current thread for a defined time.
@@ -46,7 +47,17 @@ inline bool StartNewThread(std::thread *thr, const char *name, TFn&& _Fx, TArgs&
 {
 #ifndef NO_THREADS
 	try {
+		static std::mutex thread_startup_mutex;
+		std::lock_guard<std::mutex> lock(thread_startup_mutex);
+
 		std::thread t([] (const char *name, TFn&& F, TArgs&&... A) {
+				/* Delay starting the thread till the main thread is finished
+				 * with the administration. This prevent race-conditions on
+				 * startup. */
+				{
+					std::lock_guard<std::mutex> lock(thread_startup_mutex);
+				}
+
 				SetCurrentThreadName(name);
 				CrashLog::InitThread();
 				try {