Windows: Trigger StopThread() on shutdown.

Otherwise, we end up hanging in the loop waiting for emuThreadState to
become STOPPED.  It actually has, but ThreadFrame() will block until a new
frame is queued... while will never happen, because the emuthread has
stopped.

Common/GraphicsContext.h

@@ -32,6 +32,7 @@ public:
 	virtual void ThreadStart() {}
 	virtual bool ThreadFrame() { return true; }
 	virtual void ThreadEnd() {}
+	virtual void StopThread() {}
 
 	virtual Draw::DrawContext *GetDrawContext() = 0;
 };

Windows/EmuThread.cpp

@@ -86,6 +86,9 @@ static void EmuThreadFunc(GraphicsContext *graphicsContext) {
 	emuThreadState = (int)EmuThreadState::STOPPED;
 
 	NativeShutdownGraphics();
+
+	// Ask the main thread to stop.  This prevents a hang on a race condition.
+	graphicsContext->StopThread();
 }
 
 static void EmuThreadStart(GraphicsContext *graphicsContext) {

Windows/GPU/WindowsGLContext.cpp

@@ -432,3 +432,8 @@ bool WindowsGLContext::ThreadFrame() {
 void WindowsGLContext::ThreadEnd() {
 	renderManager_->ThreadEnd();
 }
+
+void WindowsGLContext::StopThread() {
+	renderManager_->WaitUntilQueueIdle();
+	renderManager_->StopThread();
+}

Windows/GPU/WindowsGLContext.h

@@ -29,6 +29,7 @@ public:
 	void ThreadStart() override;
 	void ThreadEnd() override;
 	bool ThreadFrame() override;
+	void StopThread() override;
 
 	Draw::DrawContext *GetDrawContext() override { return draw_; }