feat: Add an SDL example to the C bindings

bindings/c/examples/sdl/.gitignore

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+build

bindings/c/examples/sdl/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.20)
+
+project(sdl_example)
+
+find_package(SDL2 REQUIRED)
+include_directories(${SDL2_INCLUDE_DIRS})
+find_package(ACCESSKIT REQUIRED)
+include_directories(windows_example ${ACCESSKIT_INCLUDE_DIR})
+
+add_executable(hello_world hello_world.c)
+target_link_libraries(hello_world PUBLIC ${SDL2_LIBRARIES})
+target_link_libraries(hello_world PUBLIC accesskit)
+target_compile_definitions(hello_world PRIVATE -DUNICODE -D_UNICODE)

bindings/c/examples/sdl/README.md

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+# AccessKit cross-platform SDL example
+
+This example demonstrates how to make use of the C bindings to create cross-platform applications.
+
+## Building
+
+The process will vary based on your operating system.
+
+### Windows:
+
+First download an SDL2 development package from the project's [GitHub release page](https://github.com/libsdl-org/SDL/releases) (SDL2-devel-2.x.y-VC.zip for MSVC) and extract it.
+
+```bash
+cmake -S . -B build -DACCESSKIT_DIR="../.." -DSDL2_DIR="<PATH_TO_SDL2_PACKAGE>/cmake"
+cmake --build build --config Release
+```
+
+You will then need to copy `SDL2.dll` into the `build/Release` folder.
+
+### Linux
+
+Make sure to install SDL2 and its development package.
+
+```bash
+cmake -S . -B build -DACCESSKIT_DIR="../.." -DCMAKE_BUILD_TYPE=Release
+cmake --build build
+```
+
+### macOS
+
+First download an SDL2 package from the project's [GitHub release page](https://github.com/libsdl-org/SDL/releases) (SDL2-2.x.y.dmg) and copy `SDL2.framework` to `/Library/Frameworks`.
+
+```bash
+cmake -S . -B build -DACCESSKIT_DIR="../.." -DCMAKE_BUILD_TYPE=Release
+cmake --build build
+```