Big example binding refactor (manually imported from Viewport branch, stripped out of Viewport code). (#1870)

Read examples/README.txt for some details.
ImDrawData: Added DisplayPos, DisplaySize fields honored by all backends (not strictly necessary to honor just now, but doing it to be ahead)

examples/imgui_impl_dx12.h

@@ -0,0 +1,30 @@
+// ImGui Win32 + DirectX12 binding
+// FIXME: 64-bit only for now! (Because sizeof(ImTextureId) == sizeof(void*))
+
+// Implemented features:
+//  [X] User texture binding. Use 'D3D12_GPU_DESCRIPTOR_HANDLE' as ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See main.cpp for an example of using this.
+// If you use this binding you'll need to call 4 functions: ImGui_ImplXXXX_Init(), ImGui_ImplXXXX_NewFrame(), ImGui::Render() and ImGui_ImplXXXX_Shutdown().
+// If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
+// https://github.com/ocornut/imgui
+
+enum DXGI_FORMAT;
+struct ID3D12Device;
+struct ID3D12GraphicsCommandList;
+struct D3D12_CPU_DESCRIPTOR_HANDLE;
+struct D3D12_GPU_DESCRIPTOR_HANDLE;
+
+// cmd_list is the command list that the implementation will use to render imgui draw lists.
+// Before calling the render function, caller must prepare cmd_list by resetting it and setting the appropriate 
+// render target and descriptor heap that contains font_srv_cpu_desc_handle/font_srv_gpu_desc_handle.
+// font_srv_cpu_desc_handle and font_srv_gpu_desc_handle are handles to a single SRV descriptor to use for the internal font texture.
+IMGUI_API bool  ImGui_ImplDX12_Init(ID3D12Device* device, int num_frames_in_flight, DXGI_FORMAT rtv_format,
+                                    D3D12_CPU_DESCRIPTOR_HANDLE font_srv_cpu_desc_handle, D3D12_GPU_DESCRIPTOR_HANDLE font_srv_gpu_desc_handle);
+IMGUI_API void  ImGui_ImplDX12_Shutdown();
+IMGUI_API void  ImGui_ImplDX12_NewFrame(ID3D12GraphicsCommandList* cmd_list);
+IMGUI_API void  ImGui_ImplDX12_RenderDrawData(ImDrawData* draw_data);
+
+// Use if you want to reset your rendering device without losing ImGui state.
+IMGUI_API void  ImGui_ImplDX12_InvalidateDeviceObjects();
+IMGUI_API bool  ImGui_ImplDX12_CreateDeviceObjects();