Send 0db ReplayGain tag as part of metadata when using replaygain_* commands with httpd output

[sevmonster]

Feature request

Most of my music players are set up to have -11.* gain for tracks that are missing ReplayGain information. But with MPD as the one managing ReplayGain, I end up getting double loss since both MPD and the client apply RG. Sending a RG value of 0db along with the other tags would prevent this from happening. I quickly scanned the documentation and source and did not see this as a feature.

[MaxKellermann]

How would that work, technically?

[sevmonster]

I don't know the specifics of the ReplayGain spec, but wouldn't it be enough to set these tags in addition to the title, artist, etc.?

REPLAYGAIN_TRACK_GAIN = "0.00 dB"
REPLAYGAIN_TRACK_PEAK = "1.000000"
REPLAYGAIN_ALBUM_GAIN = "0.00 dB"
REPLAYGAIN_ALBUM_PEAK = "1.000000"

...where the _GAIN values are attenuation (so 0 dB is no change) and _PEAK values of 1 can be interpreted in a few different ways... But should hopefully be primarily interpreted as no attenuation because the signal is not clipped and not too low... Like I said, I don't know much about RG, so these are just assumptions.