workspace: Scale down wallpaper by a fixed number of pixels

workspace: Scale down wallpaper by a fixed number of pixels

To ensure the workspace thumbnails are vertically closer to the window
picker than to the search, scale down the wallpapers by a fixed number
of pixels. Using 24 px for this means we'll take of 12 px at the top and
12 px at the bottom of the wallpaper, that's a better strategy than
always scaling it by a fixed factor since it doesn't change with the
monitor size.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1702>