vfb: allow to set any kind of resolution

Instead of limiting the size of the frame buffer, allow to allocate
a bigger frame buffer. This is useful if you have an initial one
that is small and you want to request on the fly a new one that
is bigger without the need to create a new server.
3 jobs for vfb-resolution in 8 minutes and 39 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #727191

00:01:02

 
  Build And Test
passed autotools-build-and-test #727192

00:07:36

passed meson-build-and-test #727193

00:02:27