Rather than modify the transform of the parent (which could change
independently), this adds a new override element_transform() where
element specific tranfroms can be applied. This will always stay in
sync with the attributes.
A ref test comparing a .svg and .html version of the same file is
added as due to differences in attribute parsing order, the .svg version
was previously drawn incorrectly.
Fixes#20859
I'm about to make StackingContext traverse the paintable tree instead of
actually traversing the layout tree, and it turns out we were not
creating paintables for these SVG elements.
Also switch them to Layout::Box instead of the default InlineNode to
make the trees look a bit less weird. Ultimately, we should do something
specialized for these subtrees, but for now this'll do.
