Fix repeat mode broken after first iteration (#57)

Fix the repeat mode being broken after the first iteration due to
`AnimClock::progress()` reporting a progress greater than `1.`, which was
breaking the logic of `Tween` and `Lens`.

Also fix `Tween::rewind()` not restoring the original tween direction when
using a repeat strategy of `RepeatStrategy::MirroredRepeat`.

Fixes #42