Avoid recursion in gtk_css_node_ensure_style()

Avoid recursion in gtk_css_node_ensure_style()

gtk_css_node_ensure_style() recurses over previous siblings to ensure
these have a style before its following sibling.  As seen in
https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/2027 this can
cause us to stack overflow and crash if we have a lot of children.

And even if we don't have *that* many children its still somewhat
bad to have stack depths of the same magnitude as the number of
children, both for performance reasons and debuggability.