Fix clipping with Opus files without ReplayGain tags when there's less than 5 dB headroom

[horazont]

Kindly see commit messages for what & why.

Here's some reproducer info:

Reproducer file

1. Open audacity and make sure the project sample rate is set to 48 kHz
2. Generate -> Tone...
   • Waveform = Sine
   • Frequency = 440 Hz
   • Amplitude = 1
   • Duration = 1m
3. File -> Export Audio
   • Channels = mono
   • Sample rate = 48000 Hz
   • Encoding = signed 16-bit PCM
4. Run opusenc on the resulting file

The file is also available here for download: https://sotecware.net/files/persistent/mpd-clipping.opus

Reproducer config

cat ~/.config/mpd/mpd.conf

music_directory "/home/horazont/tmp/music"
playlist_directory "/home/horazont/.local/share/mpd/playlists"
log_file "/home/horazont/.local/share/mpd/log.txt"
pid_file "/run/user/1000/mpd.pid"
state_file "/home/horazont/.config/mpd/state"
sticker_file "/home/horazont/.config/mpd/sticker.sql"
bind_to_address "::"
zeroconf_enabled "yes"
zeroconf_name "Music Player"
input {
	plugin "curl"
}

audio_output {
	type	"pulse"
	name	"Default"
	mixer_type "software"
}

replaygain "auto"

filesystem_charset "UTF-8"
id3v1_encoding	"UTF-8"

Replay gain must be enabled and mpd's volume must be at 100% for the effect to be noticeable.

Let me know if you want more of the reproducer info included in the commit message.