We were always rendering <symbol> SVG elements, but we should only
render them if they are a child of a <use>'s shadow root. This caused
practically all instances of <symbol> to be drawn at least one time too
many.
When an element has `display: contents` and it gets marked for a layout
tree rebuild, we actually have to mark its parent for rebuild as well.
The structure of the parent (and siblings) may change depending on how
the `display: contents` element changes (e.g position, display, etc.)
Otherwise the layout tree will still contain the top layer element(s).
Fixes Steam Events & Announcements `<dialog>` modal visually not fully
disappearing upon removal.
When marking a part of the layout tree for rebuild, if the subtree root
that we're marking has an anonymous parent, we now mark from the nearest
non-anonymous ancestor instead.
This ensures that subtrees inside anonymous wrappers don't just get
duplicated (i.e recreated but inserted instead of replaced).
