vfb: set gamma size to avoid xrandr to complain about it missing

3 jobs for vfb-improvements in 8 minutes and 45 seconds (queued for 25 seconds)
Status Job ID Name Coverage
  Docker Image
passed #427873
debian-testing

00:00:34

 
  Build And Test
passed #427874
autotools-build-and-test

00:07:56

passed #427875
meson-build-and-test

00:01:54