af_hdcd: Add analyze mode

A new mode, selected by filter option, to aid in analysis of HDCD
encoded audio. In this mode the audio is replaced by a solid tone and
the amplitude is adjusted to signal some specified aspect of the process.
The output file can be loaded in an audio editor alongside the original,
where the user can see where different features or states are present.
Signed-off-by: Burt P <pburt0@gmail.com>

doc/filters.texi

@@ -8356,6 +8356,38 @@ ffmpeg -i HDCD16.wav -af hdcd OUT16.wav
 ffmpeg -i HDCD16.wav -af hdcd -acodec pcm_s24le OUT24.wav
 @end example
 
+The filter accepts the following options:
+
+@table @option
+@item process_stereo
+Process the stereo channels together. If target_gain does not match between
+channels, consider it invalid and use the last valid target_gain.
+
+@item force_pe
+Always extend peaks above -3dBFS even if PE isn't signaled.
+
+@item analyze_mode
+Replace audio with a solid tone and adjust the amplitude to signal some
+specific aspect of the decoding process. The output file can be loaded in
+an audio editor alongside the original to aid analysis.
+
+@code{analyze_mode=pe:force_pe=1} can be used to see all samples above the PE level.
+
+Modes are:
+@table @samp
+@item 0, off
+Disabled
+@item 1, lle
+Gain adjustment level at each sample
+@item 2, pe
+Samples where peak extend occurs
+@item 3, cdt
+Samples where the code detect timer is active
+@item 4, tgm
+Samples where the target gain does not match between channels
+@end table
+@end table
+
 @section hflip
 
 Flip the input video horizontally.