Added SDL_GetSandbox()

This adds support for detecting whether you're running in a sandbox on macOS

CMakeLists.txt

@@ -1726,7 +1726,6 @@ elseif(UNIX AND NOT APPLE AND NOT RISCOS AND NOT HAIKU)
     sdl_sources(
       "${SDL3_SOURCE_DIR}/src/core/linux/SDL_evdev_capabilities.c"
       "${SDL3_SOURCE_DIR}/src/core/linux/SDL_threadprio.c"
-      "${SDL3_SOURCE_DIR}/src/core/linux/SDL_sandbox.c"
     )
 
     # src/core/unix/*.c is included in a generic if(UNIX) section, elsewhere.