VIDEO: Defensive code.

OS thread scheduling is variable and non-determinisitic. Defensive code
to check that startup_state isn't STARTUP_COMPLETE before waiting for
sdl_intialized_cond.

- main() executes the deferred startup, but the simulator thread hasn't
  been scheduled and isn't (yet) waiting for the sdl_initialized_cond
  condition to signal.

- STARTUP_COMPLETE is a reasonable indicator to the simulator thread
  that waiting for sdl_intialized_cond is not useful.

Bracket the entirety of the function with sdl_initialized_mutex.

sim_video.c

@@ -911,19 +911,25 @@ static void maybe_event_only_initialization()
 
 static void do_deferred_initialization()
 {
-    enum DeferredStartup_enum current_state = SDL_AtomicGet(&startup_state);
-
-    if (current_state != STARTUP_COMPLETE) {
-        SDL_LockMutex(sdl_initialized_mutex);
+    SDL_LockMutex(sdl_initialized_mutex);
 
+    if (SDL_AtomicGet(&startup_state) != STARTUP_COMPLETE) {
         /* Signal the waiting main thread */
         if (main_event_cond != NULL)
             SDL_CondSignal(main_event_cond);
 
-        /* Wait for the signal that main() is in the event queue... */
-        SDL_CondWait(sdl_initialized_cond, sdl_initialized_mutex);
-        SDL_UnlockMutex(sdl_initialized_mutex);
+        /* Wait for the signal that main() is in the event queue...
+         *
+         * Defensive code: OS thread scheduling can prevent us from receiving
+         * the condition variable's signal, i.e., main() executes the
+         * SDL_CondSignal(), but we're not actually waiting for it yet.
+         */
+        while (SDL_AtomicGet(&startup_state) != STARTUP_COMPLETE) {
+            SDL_CondWait(sdl_initialized_cond, sdl_initialized_mutex);
+        }
     }
+
+    SDL_UnlockMutex(sdl_initialized_mutex);
 }
 
 static int initialize_deferred_state()