gsdx: Don't use separate count variable

It's only ever updated after the queue is updated, so its state will
always lag slightly behind it. It's sufficient to just use empty().

This seems to fix some caching issues that were noticeable on Skylake
CPUs (#998).

plugins/GSdx/GSThread_CXX11.h

@@ -29,7 +29,6 @@ template<class T, int CAPACITY> class GSJobQueue final
 private:
 	std::thread m_thread;
 	std::function<void(T&)> m_func;
-	std::atomic<int32_t> m_count;
 	std::atomic<bool> m_exit;
 	ringbuffer_base<T, CAPACITY> m_queue;
 
@@ -43,42 +42,30 @@ private:
 
 		while (true) {
 
-			while (m_count == 0) {
+			while (m_queue.empty()) {
 				if (m_exit.load(memory_order_relaxed))
 					return;
 
 				m_notempty.wait(l);
 			}
 
-			int32_t nb = m_count;
-
 			l.unlock();
 
-			int32_t consumed = 0;
-			for (; nb >= 0; nb--) {
-				if (m_queue.consume_one(*this))
-					consumed++;
-			}
-
-			l.lock();
-
-			int32_t old_count = m_count.fetch_sub(consumed);
-
-			if (old_count == consumed) {
+			while (m_queue.consume_one(*this))
+				;
 
 			{
 				std::lock_guard<std::mutex> wait_guard(m_wait_lock);
 			}
 			m_empty.notify_one();
-			}
 
+			l.lock();
 		}
 	}
 
 public:
 	GSJobQueue(std::function<void(T&)> func) :
 		m_func(func),
-		m_count(0),
 		m_exit(false)
 	{
 		m_thread = std::thread(&GSJobQueue::ThreadProc, this);
@@ -95,34 +82,31 @@ public:
 		m_thread.join();
 	}
 
-	bool IsEmpty() const {
-		ASSERT(m_count >= 0);
-
-		return m_count == 0;
+	bool IsEmpty()
+	{
+		return m_queue.empty();
 	}
 
 	void Push(const T& item) {
 		while(!m_queue.push(item))
 			std::this_thread::yield();
 
-		std::unique_lock<std::mutex> l(m_lock);
-
-		m_count++;
-
-		l.unlock();
-
+		{
+			std::lock_guard<std::mutex> l(m_lock);
+		}
 		m_notempty.notify_one();
 	}
 
-	void Wait() {
-		if (m_count > 0) {
-			std::unique_lock<std::mutex> l(m_wait_lock);
-			while (m_count > 0) {
-				m_empty.wait(l);
-			}
-		}
+	void Wait()
+	{
+		if (IsEmpty())
+			return;
 
-		ASSERT(m_count == 0);
+		std::unique_lock<std::mutex> l(m_wait_lock);
+		while (!IsEmpty())
+			m_empty.wait(l);
+
+		assert(IsEmpty());
 	}
 
 	void operator() (T& item) {