time -> pixel -> time is not data-safe

```js
expect(instance.time(instance.at_time(0))).to.equal(0, '0 -> 0');
expect(instance.time(instance.at_time(0.15))).to.equal(0.15, '0.15 -> 0.15');
expect(instance.time(instance.at_time(1))).to.equal(1, '1 -> 1');
```