When background-size specifies only a width, the missing height was
computed as `(CSSPixels(1) / ratio) * width`. The intermediate
`CSSPixels(1) / ratio` step truncates the inverse ratio and loses
enough precision to produce a visible error in the final height for some
ratios.
Now we do the division in one step, so the full precision is preserved.
When an image has no intrinsic dimensions but has an intrinsic aspect
ratio, the CSS default sizing algorithm should resolve its size as a
contain constraint against the default object size. Previously, we
returned the default size directly, which caused such images to stretch
to fill the entire background positioning area. The SVG's default
`preserveAspectRatio` would then center the content within that
oversized viewport, making the image appear horizontally mispositioned.
