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.

examples/example_sdl3_surface/README.md

@@ -0,0 +1,33 @@
+# How to Build
+
+## Windows with Visual Studio's IDE
+
+Use the provided project file (`example_sdl3_surface.vcxproj`) or open `imgui_examples.sln`.
+
+## Windows with Visual Studio's CLI
+
+Use `build_win32.bat` or directly:
+```bat
+set SDL3_DIR=path_to_your_sdl3_folder
+cl /Zi /MD /utf-8 /I.. /I..\.. /I%SDL3_DIR%\include main.cpp ..\..\backends\imgui_impl_sdl3.cpp ..\..\backends\imgui_impl_sdlsurface3.cpp ..\..\imgui*.cpp /FeDebug/example_sdl3_surface.exe /FoDebug/ /link /libpath:%SDL3_DIR%\lib\x86 SDL3.lib /subsystem:console
+cl /Zi /MD /utf-8 /I.. /I..\.. /I%SDL3_DIR%\include main.cpp ..\..\backends\imgui_impl_sdl3.cpp ..\..\backends\imgui_impl_sdlsurface3.cpp ..\..\imgui*.cpp /FeDebug/example_sdl3_surface.exe /FoDebug/ /link /libpath:%SDL3_DIR%\lib\x64 SDL3.lib /subsystem:console
+```
+
+## Linux and similar Unixes
+
+Use the provided `Makefile` or directly:
+```sh
+c++ `pkg-config --cflags sdl3` -I .. -I ../.. -I ../../backends \
+  main.cpp ../../backends/imgui_impl_sdl3.cpp ../../backends/imgui_impl_sdlsurface3.cpp ../../imgui*.cpp \
+  -ldl `pkg-config --libs sdl3`
+```
+
+## macOS
+
+Use the provided `Makefile` or directly:
+```sh
+brew install sdl3
+c++ `pkg-config --cflags sdl3` -I .. -I ../.. -I ../../backends \
+  main.cpp ../../backends/imgui_impl_sdl3.cpp ../../backends/imgui_impl_sdlsurface3.cpp ../../imgui*.cpp \
+  -framework Cocoa -framework IOKit -framework CoreVideo `pkg-config --libs sdl3`
+```