Fixed Player::skip_one not updating player's length immediately.

[nieboczek]

Fixes #497.
Needed to add Done::should_decrement to achieve this fix.

Instead of updating sound_count in the periodic access source, I made it update directly in the skip_one function (clear affected too).
When controls.to_clear > 0 in periodic access, it will use Done::should_decrement to temporarily disable decrementing sound_count.

[nieboczek]

Something very bad seems to be happening with the Player, adding a simple assert_eq hangs the test on Player::clear:

        // ...
        assert_eq!(player.len(), 1);

        assert_eq!(source.next(), Some(1.0));

        player.clear();
        // ...

Could I get some help on this? Fixed.

[nieboczek]

I've hopefully fixed that. Also saw a queue issue where it wouldn't play the first audio appended in my own app with the version from the PR, but that is working correctly after my latest commit.