Doing multiple `for_each_in_subtree()` passes was kind of a hack. We
can resolve everything in a single pass with a little more control over
the layout process. This also fixes a few minor issues like the sizing
of nested `<g>` elements.
More work is needed here though as this is still fairly ad-hoc.
Note: This does regress `css-namespace-tag-name-selector.html`,
previously SVG text within `<a>` elements would appear. However, this
was only because `for_each_in_subtree()` would blindly look through the
InlineNodes from the unimplemented `SVGAElement`s.
Previously, we were handling viewBoxes/viewports in a slightly hacky
way, asking graphics elements to figure out what viewBox to use during
layout. This does not work in all cases, and can't allow for more
complex SVGs where it is possible to have nested viewports.
This commit makes the SVGFormattingContext keep track of the
viewport/boxes, and it now lays out each viewport recursively, where
each nested `<svg>` or `<symbol>` can establish a new viewport.
This fixes some previous edge cases, and starts to allow nested
viewports (there's still some issues to resolve there).
Fixes#22931
