quickpreview: Add some headroom when sizing the preview

Since quick preview does only integer scaling, it may be
preferable to do slight upscaling when presenting the debayered
image rather than downscaling by almost 2x, so we can save some
resources.
8 jobs for prev in 4 minutes and 32 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Package
passed #360101
librem5
package-pureos-amber

00:01:28

passed #360102
librem5:arm64
package-pureos-amber:arm64

00:04:32

passed #360103
librem5
package-pureos-byzantium

00:01:29

passed #360104
librem5:arm64
package-pureos-byzantium:arm64

00:04:20

 
  Test Package
passed #360105
librem5
autopkgtest-pureos-amber-package

00:00:41

passed #360107
librem5
autopkgtest-pureos-byzantium-package

00:00:55

passed #360106
librem5
lintian-pureos-amber-package

00:00:38

passed #360108
librem5
lintian-pureos-byzantium-package

00:00:47