Fix ub and shrink unsafe blocks

[antonilol]

Breaking changes:

• mixer::Chunk fields were made private because the struct relied on these pointer being valid, but users could set them to any value
• Timer callback must live for 'static because the callback can be called after the scope ends (by using std::mem::forget).

timer ub reproduction (ran from a #[test]):

let sdl_context = crate::sdl::init().unwrap();
let timer_subsystem = sdl_context.timer().unwrap();

let mut v = Box::new(123);

std::mem::forget(timer_subsystem.add_timer(
    20,
    Box::new(|| {
        dbg!(std::mem::take(&mut v));
        0
    }),
));

drop(v);

// v is used after dropping (from the timer callback)

std::thread::sleep(Duration::from_millis(30));

[antonilol]

I don't know why that did not work, it should work.

Since Rust 1.81, unwinding in an extern "C" function will abort the process, I added the catch_unwind to not have this make undefined behavior on "older" (1.81 is almost 2 weeks old at the time of writing this comment) versions and have the same behavior independent of the Rust version.