Layer shapes are wrong

As reported in #vello > Servo 2D canvas backend @ 💬 vello currently chooses to big size (and they are also off) for layers, I suspect something is wrong with binning.

For this code (also available as smoke test https://github.com/linebender/vello/pull/1062):

scene.fill(
        vello::peniko::Fill::NonZero,
        Affine::IDENTITY,
        vello::peniko::color::AlphaColor::from_rgb8(0, 255, 0),
        None,
        &Rect::from_origin_size((0.0, 0.0), (60., 60.)),
    );
    scene.push_layer(
        vello::peniko::Compose::Clear,
        1.0,
        vello::kurbo::Affine::IDENTITY,
        &Rect::from_origin_size((20.0, 20.0), (20., 20.)),
    );
    scene.pop_layer();
    scene.fill(
        vello::peniko::Fill::NonZero,
        Affine::IDENTITY,
        vello::peniko::color::AlphaColor::from_rgb8(255, 0, 0),
        None,
        &Rect::from_origin_size((20.0, 20.0), (20., 20.)),
    );

one would expect to get

but what we actually get is

Start with the layer reproduction shown in the issue and the smoke test linked in PR #1062. Compare the expected and actual output, then trace the layer binning path to determine why the layer is oversized and offset. Done means the example renders the expected green background with the cleared central layer and red shape.