shadow-helper: Don't set style context parent

Even though foreign drawing uses a temporary GtkStyleContext, calling
gtk_style_context_set_parent() adds all its CSS nodes to the global
hierarchy, and then they aren't cleaned up. This shouldn't happen.

Fixes Librem5/libhandy#191
12 jobs for wip/exalm/nodes in 22 minutes and 19 seconds (queued for 23 seconds)
Status Job ID Name Coverage
  Build
passed #240235
librem5
api-visibility

00:02:46

passed #240238
librem5
build-debian-clang

00:04:19

passed #240236
librem5
build-debian-gcc

00:03:16

passed #240237
librem5
build-debian-gcc-static

00:03:07

passed #240239
librem5
build-fedora-gcc

00:03:09

 
  Test+Docs
passed #240240
librem5
build-gtkdoc

00:02:55

passed #240241
librem5
unit-test

00:03:26

43.8%
 
  Package
passed #240242
librem5
package-debian-buster

00:03:41

passed #240243
librem5:arm64
package-debian-buster:arm64

00:05:43

passed #240244
flatpak
package-flatpak

00:01:51

 
  Test Package
passed #240245
librem5
autopkgtest-debian-buster-package

00:02:54

passed #240246
librem5
lintian-debian-buster-package

00:01:10