Previously, any SVG geometry attribute change would mark the entire
document layout tree as dirty, triggering a full layout pass even though
only the SVG subtree was affected. This made SVG geometry animations
unnecessarily expensive.
Fix this by stopping `needs_layout_update` propagation at the SVGSVGBox
boundary and tracking dirty SVG roots separately on the Document. When
`update_layout()` finds that only SVG roots need relayout (and the
document layout root is clean), it runs SVGFormattingContext on each
dirty SVG root in a fresh LayoutState and commits the results directly,
bypassing the full document layout pass entirely.
This results in a substantial performance improvement on pages with
animated SVGs, such as https://www.cloudflare.com/,
https://www.duolingo.com/, and our GC graph explorer page.
abecc746d7