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/Makefile

@@ -0,0 +1,53 @@
+EXE = example_sdl3_surface
+IMGUI_DIR = ../..
+SOURCES = main.cpp
+SOURCES += $(IMGUI_DIR)/imgui.cpp $(IMGUI_DIR)/imgui_demo.cpp $(IMGUI_DIR)/imgui_draw.cpp $(IMGUI_DIR)/imgui_tables.cpp $(IMGUI_DIR)/imgui_widgets.cpp
+SOURCES += $(IMGUI_DIR)/backends/imgui_impl_sdl3.cpp $(IMGUI_DIR)/backends/imgui_impl_sdlsurface3.cpp
+OBJS = $(addsuffix .o, $(basename $(notdir $(SOURCES))))
+UNAME_S := $(shell uname -s)
+
+CXXFLAGS = -std=c++11 -I$(IMGUI_DIR) -I$(IMGUI_DIR)/backends
+CXXFLAGS += -g -Wall -Wformat
+LIBS =
+
+ifeq ($(UNAME_S), Linux)
+	ECHO_MESSAGE = "Linux"
+	LIBS += -ldl `pkg-config sdl3 --libs`
+	CXXFLAGS += `pkg-config sdl3 --cflags`
+	CFLAGS = $(CXXFLAGS)
+endif
+
+ifeq ($(UNAME_S), Darwin)
+	ECHO_MESSAGE = "Mac OS X"
+	LIBS += -framework Cocoa -framework IOKit -framework CoreVideo
+	LIBS += `pkg-config --libs sdl3`
+	LIBS += -L/usr/local/lib -L/opt/local/lib
+	CXXFLAGS += `pkg-config --cflags sdl3`
+	CXXFLAGS += -I/usr/local/include -I/opt/local/include
+	CFLAGS = $(CXXFLAGS)
+endif
+
+ifeq ($(OS), Windows_NT)
+	ECHO_MESSAGE = "MinGW"
+	LIBS += -lgdi32 -limm32 `pkg-config --static --libs sdl3`
+	CXXFLAGS += `pkg-config --cflags sdl3`
+	CFLAGS = $(CXXFLAGS)
+endif
+
+%.o:%.cpp
+	$(CXX) $(CXXFLAGS) -c -o $@ $<
+
+%.o:$(IMGUI_DIR)/%.cpp
+	$(CXX) $(CXXFLAGS) -c -o $@ $<
+
+%.o:$(IMGUI_DIR)/backends/%.cpp
+	$(CXX) $(CXXFLAGS) -c -o $@ $<
+
+all: $(EXE)
+	@echo Build complete for $(ECHO_MESSAGE)
+
+$(EXE): $(OBJS)
+	$(CXX) -o $@ $^ $(CXXFLAGS) $(LIBS)
+
+clean:
+	rm -f $(EXE) $(OBJS)