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.

docs/EXAMPLES.md

@@ -145,6 +145,10 @@ SDL2 (Win32, Mac, Linux, etc.) + SDL_Renderer for SDL2 example.<BR>
 = main.cpp + imgui_impl_sdl2.cpp + imgui_impl_sdlrenderer2.cpp <BR>
 This requires SDL 2.0.18+ (released November 2021) <BR>
 
+[example_sdl2_surface/](https://github.com/ocornut/imgui/blob/master/examples/example_sdl2_surface/) <BR>
+SDL2 (Win32, Mac, Linux, etc.) + SDL_Surface CPU renderer example.<BR>
+= main.cpp + imgui_impl_sdl2.cpp + imgui_impl_sdlsurface2.cpp <BR>
+
 [example_sdl2_vulkan/](https://github.com/ocornut/imgui/blob/master/examples/example_sdl2_vulkan/) <BR>
 SDL2 (Win32, Mac, Linux, etc.) + Vulkan example. <BR>
 = main.cpp + imgui_impl_sdl2.cpp + imgui_impl_vulkan.cpp <BR>
@@ -178,6 +182,10 @@ SDL3 (Win32, Mac, Linux, etc.) + SDL_GPU for SDL3 example.<BR>
 SDL3 (Win32, Mac, Linux, etc.) + SDL_Renderer for SDL3 example.<BR>
 = main.cpp + imgui_impl_sdl3.cpp + imgui_impl_sdlrenderer3.cpp <BR>
 
+[example_sdl3_surface/](https://github.com/ocornut/imgui/blob/master/examples/example_sdl3_surface/) <BR>
+SDL3 (Win32, Mac, Linux, etc.) + SDL_Surface CPU renderer example.<BR>
+= main.cpp + imgui_impl_sdl3.cpp + imgui_impl_sdlsurface3.cpp <BR>
+
 [example_sdl3_vulkan/](https://github.com/ocornut/imgui/blob/master/examples/example_sdl3_vulkan/) <BR>
 SDL3 (Win32, Mac, Linux, etc.) + Vulkan example. <BR>
 = main.cpp + imgui_impl_sdl3.cpp + imgui_impl_vulkan.cpp <BR>
@@ -253,4 +261,3 @@ when an interactive drag is in progress.
 Note that some setup configurations or GPU drivers may introduce additional display lag depending on their settings.
 If you notice that dragging windows is laggy and you are not sure what the cause is: try drawing a simple
 2D shape directly under the mouse cursor to help identify the issue!
-