cssnode: Don't put wrong styles in the style cache

cssnode: Don't put wrong styles in the style cache

~Company ╡ so TL;DR: we put the static style in the cache, but then
       ⤷ ╡ compute a child style from the animated style in the cache
       ⤷ ╡ and we put the child style also in the cache (because
       ⤷ ╡ it's not animated)
       ⤷ ╡ then we run the animation, but reuse the cache every time
       ⤷ ╡ for both child and parent
       ⤷ ╡ so after the animation is done, we end up with a cache that
       ⤷ ╡ has the correct static style for the parent but an
       ⤷ ╡ incorrect static style for the child
       ⤷ ╡ because that static style was computed from the
       ⤷ ╡ initial animated style

This fixes https://bugzilla.gnome.org/show_bug.cgi?id=763517