Support another Electronic Arts format

[Carl Eugen Hoyos]

Another type of ea samples was uploaded by an irc user, contains mpegvideo and adpcm (?) audio, file type is MGAV.

$ ffmpeg -i EAS.PS2
ffmpeg version N-92718-g092cb17 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.4.0 (GCC)
  configuration: --enable-gpl --enable-libxml2 --enable-gnutls --enable-gmp --enable-version3
  libavutil      56. 24.101 / 56. 24.101
  libavcodec     58. 42.102 / 58. 42.102
  libavformat    58. 24.101 / 58. 24.101
  libavdevice    58.  6.101 / 58.  6.101
  libavfilter     7. 46.101 /  7. 46.101
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
[mpeg1video @ 0x434ac80] Invalid frame dimensions 0x0.
[mpegvideo @ 0x4349500] Estimating duration from bitrate, this may be inaccurate
Input #0, mpegvideo, from 'EAS.PS2':
  Duration: 00:00:04.32, bitrate: 4096 kb/s
    Stream #0:0: Video: mpeg2video (Main), yuv420p(tv, bt470bg/bt470m/bt470m, progressive), 512x448 [SAR 7:6 DAR 4:3], 4096 kb/s, 29.97 fps, 59.94 tbr, 1200k tbn, 59.94 tbc
At least one output file must be specified