Let us start here, from square one. No, from Zero!

overlay/imgui/impl_dx9.h

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+// dear imgui: Renderer for DirectX9
+// This needs to be used along with a Platform Binding (e.g. Win32)
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use 'LPDIRECT3DTEXTURE9' as ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
+//  [X] Renderer: Support for large meshes (64k+ vertices) with 16-bits indices.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See main.cpp for an example of using this.
+// If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
+// https://github.com/ocornut/imgui
+
+#pragma once
+
+#include "external/imgui/imgui.h"
+
+struct IDirect3DDevice9;
+
+IMGUI_IMPL_API bool     ImGui_ImplDX9_Init(IDirect3DDevice9 *device);
+IMGUI_IMPL_API void     ImGui_ImplDX9_Shutdown();
+IMGUI_IMPL_API void     ImGui_ImplDX9_NewFrame();
+IMGUI_IMPL_API void     ImGui_ImplDX9_RenderDrawData(ImDrawData *draw_data);
+
+// Use if you want to reset your rendering device without losing ImGui state.
+IMGUI_IMPL_API bool     ImGui_ImplDX9_CreateDeviceObjects();
+IMGUI_IMPL_API void     ImGui_ImplDX9_InvalidateDeviceObjects();