Shrink shadow extents

Shrink shadow extents

Long time ago, Cairo shadows in both GTK3 and 4 were drawn at a size about
twice their radius. Eventually this was fixed but the shadow extents are
still calculated for the previous size and appear unreasonably large: for
example, 141px for a 50px radius shadow. This can get very noticeable in
places such as invisible window frame which gets included into screenshots.

https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3419 just divides the
radius by 2 when drawing a shadow with Cairo, do the same when calculating
extents.

See https://gitlab.gnome.org/GNOME/gtk/-/issues/3841