This is a small convenience wrapper around clutter_color_equal()
for the different color components, which also handles the case
where one (or both) of the icon colors are %NULL.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1212
A new StIconColors object is used to efficiently track the colors
we need to colorize a symbolic icon.
st_theme_node_compute_icon_colors() is added to compute the
StIconColors for a theme node. (Refcounting of StIconColors means
that we'll typically share the colors object of the parent node.)
https://bugzilla.gnome.org/show_bug.cgi?id=633865