Previously, `Document::notify_css_background_image_loaded()` walked the
entire `PaintableBox` subtree and cleared each box's paintable cache
whenever any CSS image finished loading.
Replace this with per-image observers owned by the layout node. During
`apply_style`, each node registers as an `ImageStyleValue::Client` for
the images its style references. On load, only the affected layout
node's paintables are invalidated.
Add an abstract image style value for image-set() and parse both the
standard and -webkit-prefixed spellings through the existing <image>
value path. The parser accepts URL and string image candidates,
optional resolution descriptors, and type() filters.
Track attr-taint through substituted component values so image-set()
candidates using attr()-derived URL-producing tokens are rejected when
resolved for URL-using properties.
Update the relevant WPT baselines now that image-set() parsing is
supported in additional value contexts.
