Previously, referencing an element that eventually pointed back to
the original <use> element would cause a stack overflow.
We now look at the referenced element and follow every <use> element
in its subtree and aim to detect any duplicates. If so, we consider
that referenced element invalid.
Previously, the first `HTMLHtmlELement` in the given document would
always be used when determining whether to propagate background
properties to the body element. This meant the wrong root element was
used for SVG `foreignObject` elements, which could lead to a crash.
Previously, an SVG with width of zero would have am intrinsic aspect
ratio of zero. With this change, if an SVG has a width or height of
zero, the intrinsic aspect ratio is determined by the SVG's viewbox.
