Introduce ImGui Unity build

imgui/backends/imgui_impl_dx9.h

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+// dear imgui: Renderer Backend for DirectX9
+// This needs to be used along with a Platform Backend (e.g. Win32)
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use 'LPDIRECT3DTEXTURE9' as ImTextureID. Read the FAQ about ImTextureID!
+//  [X] Renderer: Large meshes support (64k+ vertices) with 16-bit indices.
+
+// You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
+// If you are new to Dear ImGui, read documentation from the docs/ folder + read the top of imgui.cpp.
+// Read online: https://github.com/ocornut/imgui/tree/master/docs
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+#ifndef IMGUI_DISABLE
+
+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 Dear ImGui state.
+IMGUI_IMPL_API bool     ImGui_ImplDX9_CreateDeviceObjects();
+IMGUI_IMPL_API void     ImGui_ImplDX9_InvalidateDeviceObjects();
+
+#endif // #ifndef IMGUI_DISABLE