This change is preparation to use ffmpeg's resampling capabilities to keep local
audio in sync (by up/downsampling slightly). This requires that sample rates are
not fixed for a transcode profile.
Added benefit of this is that we don't need quite as many xcode profiles.
Make it easier to add new parameters later, get rid of redundant code, clean
up, align between httpd_xxx modules and introduce new bugs. Yes, the refactor
got a bit out of hand.
rescaling, meaning we can do without parallel ffmpeg interfaces.
This also moves artwork rescaling from libswscale to libavfilter, which
seems to fix a problem with PNG rescaling.
Also includes the skeleton for perhaps supporting video in the future. Adds more fine-grained
ffmpeg/libav compability checks. Dependency on libavresample/libswresample exchanged with
dependency on libavfilter, which seems more versatile.