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.

Common/Thread/ThreadManager.h

@@ -46,6 +46,7 @@ public:
 	void Init(int numCores, int numLogicalCoresPerCpu);
 	void EnqueueTask(Task *task, TaskType taskType);
 	void EnqueueTaskOnThread(int threadNum, Task *task, TaskType taskType);
+	void Teardown();
 
 	// Currently does nothing. It will always be best-effort - maybe it cancels,
 	// maybe it doesn't. Note that the id is the id() returned by the task. You need to make that
@@ -57,8 +58,6 @@ public:
 	int GetNumLooperThreads() const;
 
 private:
-	void Teardown();
-
 	GlobalThreadContext *global_ = nullptr;
 
 	int numThreads_ = 0;