st/theme-node-drawing: Render box-shadow independently of background-color

st/theme-node-drawing: Render box-shadow independently of background-color

Until now, the absence of a `background-color` would cause `box-shadow`s
to be invisible. That was a bug because `box-shadow` is meant to be either
the color specified in `box-shadow`, or if none was specified then the
foreground `color`.

There is nothing in the spec that says the `box-shadow` rendering should
depend on `background-color`, so separate them. This ensures `box-shadow`
is rendered even when `background-color` is absent or transparent.

https://www.w3.org/TR/css-backgrounds-3/#box-shadow

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1904>