This slightly changes the customizations logic. We now make sure
that each stage is appended exactly once.
customizations.go are now responsible only for the things that are
completely generic, and not per-ouput-type. helpers.go contain more
high-level helpers that combine customziations and per-output-type
defaults.
This does not change the behaviour, though some pipelines are slightly
reordered to make them consistent.
Signed-off-by: Tom Gundersen <teg@jklm.no>
de93ddc757