Sound memory leak

[andreymal]

loop {
    load_sound("./sound.ogg").await?;
}

RAM is leaking.

Like with font atlas, I tried to implement Drop for Sound:

impl Drop for Sound {
    fn drop(&mut self) {
        if Arc::strong_count(&self.0) < 2 {
            self.0.delete(&get_context().audio_context.native_ctx);
        }
    }
}

and this fixed the leak (but I'm not sure if this solution is complete and/or correct because I don't even know why Arc is used here)

[not-fl3]

Hopefully fixed with #813