Add SDL3 surface backend for ImGui example

- Created imgui_impl_sdlsurface3.h and corresponding implementation file.
- Added Makefile for building the SDL3 surface example on Linux and macOS.
- Included README.md with build instructions for Windows, Linux, and macOS.
- Added build script for Windows (build_win32.bat) and Visual Studio project files.
- Implemented main application logic in main.cpp to demonstrate ImGui with SDL3 and surface rendering.

backends/imgui_impl_sdlsurface3.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "imgui.h"
+#include <SDL3/SDL.h>
+
+IMGUI_API bool ImGui_ImplSDLSurface3_Init(SDL_Surface* surface);
+IMGUI_API void ImGui_ImplSDLSurface3_Shutdown();
+IMGUI_API void ImGui_ImplSDLSurface3_NewFrame();
+IMGUI_API void ImGui_ImplSDLSurface3_RenderDrawData(ImDrawData* draw_data);
+
+IMGUI_API SDL_Surface* ImGui_ImplSDLSurface3_CreateFontAtlasSurface();