Rendering bug when stroking rect on window boundary

Nobody has claimed this yet.

It only happens with certain origins and widths. To reproduce, swap this function at the bottom of the `simple` example:

```rust
pub fn add_shapes_to_scene(scene: &mut Scene) {
let rect = vello::kurbo::Rect::from_origin_size((113.5, 0.5), (103.0, 61.0));
scene.stroke(&Stroke::new(1.0), Affine::IDENTITY, Color::from_rgb8(255, 0, 0), None, &rect);
}
```

Replace the function at the bottom of the `simple` example with the reproduction shown, then run the example and inspect the stroked rectangle at the window boundary. Trace the rendering path for origins and widths that reproduce the artifact; done means the rectangle renders correctly for the supplied case and other affected boundary positions.

Written by the indexing model from the issue text.