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.