st-theme-node: Move some allocation-independent textures back to the theme node

st-theme-node: Move some allocation-independent textures back to the theme node

The background image, background image shadow and border image are
allocation-indepedent, so we can keep these in the node. Given that these are
are likely cached in the StTextureCache, the slight increase in code complexity
may not be worth caching these textures and materials -- we might be better off
just computing when we need to paint.

https://bugzilla.gnome.org/show_bug.cgi?id=697274