Function retro_set_environment 

pub unsafe extern "C" fn retro_set_environment(
    arg1: Option<unsafe extern "C" fn(u32, *mut c_void) -> bool>,
)

Sets callbacks. retro_set_environment() is guaranteed to be called before retro_init().

The rest of the set_* functions are guaranteed to have been called before the first call to retro_run() is made.