Fix black screen on multiple libretro content loads.

Moves ThreadPool teardown to retro_unload_game. Gives ThreadPool threads some breathing room to terminate before game load starts spamming it with tasks.

libretro/libretro.cpp

@@ -675,6 +675,8 @@ void retro_unload_game(void)
 	delete ctx;
 	ctx = nullptr;
 	PSP_CoreParameter().graphicsContext = nullptr;
+
+   g_threadManager.Teardown();
 }
 
 void retro_reset(void)