common: SPSCQueue: Notify after incrementing queue size.

src/common/threadsafe_queue.h

@@ -46,8 +46,15 @@ public:
         ElementPtr* new_ptr = new ElementPtr();
         write_ptr->next.store(new_ptr, std::memory_order_release);
         write_ptr = new_ptr;
-        ++size;
 
+        const size_t previous_size{size++};
+
+        // Acquire the mutex and then immediately release it as a fence.
+        // TODO(bunnei): This can be replaced with C++20 waitable atomics when properly supported.
+        // See discussion on https://github.com/yuzu-emu/yuzu/pull/3173 for details.
+        if (previous_size == 0) {
+            std::lock_guard lock{cv_mutex};
+        }
         cv.notify_one();
     }