opus: do not add 5 dB to Opus header playback gain

The playback gain in the Opus header is not related to any reference
like ReplayGain or EBU R128. It thus does not need converting into the
ReplayGain "space".

This patch thus removes the 5 dB offset from the fallback path in
MPDOpusDecoder::HandleAudio (where the Opus header gain is converted
into ReplayGain metadata).

Otherwise, an Opus file without replay gain tags and with an output
gain of 0 dB would get a gain of 5 dB. This leads to clipping if the
audio data has less than 5 dB of headroom.

The clipping can be reproduced with something as simple as an amplitude
1.0 sine file. One way to create such a file is via audacity:

1. Generate -> Tone...
   - Waveform = Sine
   - Frequency = 440 Hz
   - Amplitude = 1
   - Duration = 1m
2. File -> Export Audio
   - Channels = mono
   - Sample rate = 48000 Hz
   - Encoding = signed 16-bit PCM
3. Run opusenc on the resulting file

Author: horazont

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -312,7 +312,7 @@ MPDOpusDecoder::HandleAudio(const ogg_packet &packet)
 		   gain" value */
 		ReplayGainInfo rgi;
 		rgi.Clear();
-		rgi.track.gain = EbuR128ToReplayGain(output_gain);
+		rgi.track.gain = output_gain;
 		client.SubmitReplayGain(&rgi);
 		submitted_replay_gain = true;
 	}