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.

!3419 (merged) just divides the radius by 2 when drawing a shadow with Cairo, do the same when calculating extents.

See #3841 (closed)

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Before:

After:

For comparison, GTK3 where the old extents actually match the shadow: