WebCodecs: VideoFrame(CanvasImageSource) deterministically mutates some explicit timestamps by -1 µs

```js
const canvas = document.createElement('canvas');
canvas.width = 2;
canvas.height = 2;

const frame = new VideoFrame(canvas, {
    timestamp: 1001000
});

console.log(frame.timestamp);
```

### Expected

```text
1001000
```

### Actual in Safari/WebKit

```text
1000999
```

Reproduced on macOS 26, iOS 18/26. 

More Details: https://bugs.webkit.org/show_bug.cgi?id=321880