Support USM format

[llogan]

User "Win Is Best" provided two USM files from the game "Metal Gear Rising: Revengeance".

$ ffmpeg -f mpegvideo -i ev9020.usm 
ffmpeg version N-89933-g782e066e3e Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 7.2.1 (GCC) 20171224
  configuration: 
  libavutil      56.  7.100 / 56.  7.100
  libavcodec     58.  9.100 / 58.  9.100
  libavformat    58.  7.100 / 58.  7.100
  libavdevice    58.  0.101 / 58.  0.101
  libavfilter     7. 11.101 /  7. 11.101
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
[mpeg1video @ 0x5607764ae4c0] Invalid frame dimensions 0x0.
    Last message repeated 4 times
[mpeg1video @ 0x5607764ae4c0] slice below image (55 >= 45)
[mpegvideo @ 0x5607764a4580] Estimating duration from bitrate, this may be inaccurate
Input #0, mpegvideo, from 'ev9020.usm':
  Duration: 00:00:01.84, bitrate: 104857 kb/s
    Stream #0:0: Video: mpeg1video, yuv420p(tv), 1280x720 [SAR 1:1 DAR 16:9], 104857 kb/s, 29.50 fps, 29.97 tbr, 1200k tbn, 29.97 tbc
At least one output file must be specified

Samples:

​https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket6995/

[Elon Musk]

Added in 9e1ea3caba5effaf9c7596dc178bd457439d638f