[Feature]: Add Opus replay gain support

[Amonoman]

Is your feature request related to a problem?

Opus files aren't covered by the current replay gain handling, so
they play back without loudness normalization even when gain metadata
is present in the file.

Describe the solution you'd like

Parse replay gain for Opus files from R128_TRACK_GAIN /
R128_ALBUM_GAIN Vorbis comment tags (the common case for most
encoders), falling back to Opus's built-in header output gain field
if comment tags aren't present. Convert to the same internal gain
format used for other codecs.

Describe alternatives you've considered

Header output gain alone would be simpler, but misses tags from most
common encoders (opusenc, ffmpeg), which write R128 comment tags.

[Vioddie]

I second this. Though OPUS works with standard ReplayGain tags, I believe support for R128 could enhance compatibility for libraries heavily composed with OPUS files. It would reduce the hassle of redoing your library with standard ReplayGain tags.

However, ReplayGain (with standard tags) isn’t reliable as of the current version (#2416 and #2478 are examples). On my end, it only works for negative values.