Select best cover art if multiple embedded

[cgbug]

File contains multiple covers. Wrong one is displayed in FFplay. The metadata comment value could be used as a hint which one should be preferred.

Sample:
​https://www.datafilehost.com/d/6353053c

ffmpeg version N-92079-gb577153231 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 8.2.1 (GCC) 20180813
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
  libavutil      56. 19.101 / 56. 19.101
  libavcodec     58. 31.102 / 58. 31.102
  libavformat    58. 18.103 / 58. 18.103
  libavdevice    58.  4.104 / 58.  4.104
  libavfilter     7. 33.100 /  7. 33.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Input #0, mp3, from 'Tom Staar & Corey James - Bird Flu (Extended Mix).mp3':
  Metadata:
    album           : Bird Flu [ 
                    : vk.com/speeeedy]
    artist          : Tom Staar & Corey James
    TBPM            : 126
    comment         : Speeeedy EDM Blog - главный источник электронной музыки вконтакте! 
                    : vk.com/speeeedy
    TIT1            : Cartel Music
    encoded_by      : Beatport
    TFLT            : MPG/3
    genre           : Tech House
    TKEY            : Fmaj
    TSRC            : NLZ541700206
    TORY            : 2017-05-01
    publisher       : Cartel Music
    lyrics-xxx      : Speeeedy EDM Blog - главный источник электронной музыки вконтакте! 
                    : vk.com/speeeedy
    title           : Bird Flu (Extended Mix)
    track           : 01
    date            : 2017
  Duration: 00:04:58.81, start: 0.025056, bitrate: 328 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 320 kb/s
    Metadata:
      encoder         : LAME3.97 
    Side data:
      replaygain: track gain - -10.400000, track peak - unknown, album gain - unknown, album peak - unknown, 
    Stream #0:1: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 500x500 [SAR 1:1 DAR 1:1], 90k tbr, 90k tbn, 90k tbc
    Metadata:
      comment         : Publisher/Studio logotype
    Stream #0:2: Video: png, pal8(pc), 1500x250, 90k tbr, 90k tbn, 90k tbc
    Metadata:
      comment         : A bright coloured fish
    Stream #0:3: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 1000x1000 [SAR 72:72 DAR 1:1], 90k tbr, 90k tbn, 90k tbc
    Metadata:
      comment         : Cover (front)