Store the ReplayGain tag values as strings

[robUx4]

This is how it's stored in Vorbis [1][2] and how
mkvmerge passes the value from one to the other [3]. There doesn't seem to be any player reading it from Matroska.

[1] https://code.videolan.org/videolan/vlc/-/blob/f1a90394435471e75fee6f13383079a3d0272ec4/modules/demux/xiph_metadata.c#L472
[2] https://github.com/FFmpeg/FFmpeg/blob/153a6dc8faafc4de263a493484ffc1dc2b5b26b2/libavformat/replaygain.c
[3] https://gitlab.com/mbunkus/mkvtoolnix/-/blob/7a32d31e7029b1ca4f323eb0f7fb975ec979b089/src/common/tags/vorbis.cpp#L78

I would have preferred a binary version of a float value to avoid parsing errors. But for backward compatibility it's probably better to keep it like this. Unless mkvmerge ends up turning Vorbis replay gain into a binary format ? I don't know the code enough to tell for sure, cc @mbunkus.

[robUx4]

That leaves MCDI as the only (official) binary tag. It may change with #831 but we may just use strings as for ReplayGain.

[robUx4]

I tried muxing a .ogg file with REPLAYGAIN_GAIN and REPLAYGAIN_PEAK and mkvmerge did turn them into string (or kept them as-is).

[robUx4]

But the "dB" unit is there in the gain and the peak level is not in dB.

[robUx4]

The original files has those strings:

replaygain_album_true_peak=1.117522
replaygain_track_true_peak=1.117522
replaygain_album_gain=-7.31 dB
replaygain_album_peak=1.072099
replaygain_track_gain=-7.31 dB
replaygain_track_peak=1.072099

[robUx4]

BTW, given it influences the playback (it should, in an ideal world) I think it could be elements of the TrackEntry\Audio. We would need to put the "album" and "track" values. But if the file is a whole album with tracks split as chapters, we can't define individual tracks from there...

[robUx4]

Merging this as this matches what exists in the wild.