According to the GNOME 40 mockups, add a box-shadow to the workspace
background. For this to work, we also need a background-color (St
limitations), and to make sure that background-color doesn't bleed over
the rounded corners of the wallpaper, we also need to tell St to use a
border-radius and clip the background-color painting using a rounded
path.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1651>