Add imgui-based Netplay Chat

Source/Core/VideoCommon/NetPlayChatUI.h

@@ -0,0 +1,35 @@
+// Copyright 2019 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <array>
+#include <deque>
+#include <functional>
+#include <memory>
+#include <string>
+#include <utility>
+
+class NetPlayChatUI
+{
+public:
+  explicit NetPlayChatUI(std::function<void(const std::string&)> callback);
+  ~NetPlayChatUI() = default;
+
+  using Color = std::array<float, 3>;
+
+  void Display();
+  void AppendChat(const std::string& message, Color color);
+  void SendMessage();
+
+private:
+  char m_message_buf[256] = {};
+  bool m_scroll_to_bottom = false;
+  bool m_is_scrolled_to_bottom = true;
+
+  std::deque<std::pair<std::string, Color>> m_messages;
+  std::function<void(const std::string&)> m_message_callback;
+};
+
+extern std::unique_ptr<NetPlayChatUI> g_netplay_chat_ui;