The new HDCD filter really does nothing to show that it is working or
that HDCD control information was even detected in the stream. This
patch collects information about the decode, like which features were
used, and reports it to the user at the end.

Also,
* Fixes low-level gain adjustment
* Updates the documentation
Signed-off-by: Burt P <pburt0@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>