stackable-box: Fix natural size calculation for deck

Fixes GNOME/libhandy#404
11 jobs for deck-nat in 18 minutes and 33 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #1120278

00:00:19

passed build-debian-clang #1120280

00:03:55

passed build-debian-gcc #1120279

00:02:37

passed build-fedora-gcc #1120281

00:07:21

passed build-flatpak #1120282
flatpak

00:02:25

passed build-flatpak-glade #1120283
flatpak

00:04:12

 
  Test+Docs
passed build-gtkdoc #1120284

00:03:24

passed unit-test #1120285

00:03:25

46.3%
 
  Package
passed package-debian-buster #1120286

00:03:50

 
  Test Package
passed autopkgtest-debian-buster-package #1120287

00:03:55

passed lintian-debian-buster-package #1120288

00:02:48