This removes the internal interleave loop and always negotiates
the native output layout of the libav decoder. Users can use
audioconvert to interleave if necessary.
Special care has been taken to leave the encoder unaffected by
the changes in avcodecmap, since GstAudioEncoder doesn't support
the non-interleaved layout yet.
https://bugzilla.gnome.org/show_bug.cgi?id=705977
The remaining use of CODEC_ are codec flags that has been moved into the
new codec private properties or have been deperated. Will be fixed in
later patches.
https://bugzilla.gnome.org/show_bug.cgi?id=792900