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

```js
expect(instance.at_time(instance.time(0))).to.equal(0, '0 -> 0');
expect(instance.at_time(instance.time(14))).to.equal(14, '14 -> 14');
expect(instance.at_time(instance.time(93))).to.equal(93, '93 -> 93');
```