background: Scale by fractional output scale

Scale the background by logical scale instead of what we get
from xdg_output->scale. This allows the background to scale
with fractional scaling.

Closes: #338

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
9 jobs for background-scale in 48 minutes and 31 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #359060
librem5
build:cross-debian-bullseye:i386

00:16:13

passed #359061
librem5
build:native-alpinelinux-edge

00:10:56

passed #359059
librem5
build:native-debian-bullseye

00:16:21

 
  Test+Docs
passed #359063
librem5
build-gtkdoc

00:12:25

passed #359065
librem5
check-license-headers

00:02:33

passed #359064
librem5
check-po

00:00:27

passed #359066
librem5
unit-test:native-alpinelinux-edge

00:00:50

passed #359062
librem5
unit-test:native-debian-bullseye

00:14:05

53.1%
 
  Package
passed #359067
librem5:arm64
package:deb-pureos-byzantium:arm64

00:18:04