workshop: reimplment hstack as pack

An op to put two buffers next to each other, with an optional gap.
6 jobs for master in 12 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #725217
build-image

00:01:36

 
  Dependencies
passed #725218
babl-master

00:04:44

passed #725219
babl-min

00:04:58

 
  Gegl
passed #725222
debug-fullconfig-master-babl

00:06:15

passed #725220
default-min-babl

00:04:29

passed #725221
fullconfig-master-babl

00:06:15