It spams these in verbose mode. It's caused by format negotiation code
in af.c. It's for the mpv format to ffmpeg-format case, and that one is
very uninteresting. (The ffmpeg supported audio formats are practically
never extended.)
To make this easier, get rid of the direct mapping of the
AF_FORMAT_BITS_MASK bit field to number of bytes. This way we can throw
away the unused AF_FORMAT_48BIT and don't have to add ..._56BIT.
