riff: add "SLES" FourCC as mpeg2video

[ami_stuff]

this FourCC is used by "SoftLab-NSK advanced I-frames" codec

./ffmpeg -i SoftLab-NSK_advanced_I-frames_2.avi
ffmpeg version N-109332-g45ab5307a6 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: --enable-libopenjpeg
  libavutil      57. 43.100 / 57. 43.100
  libavcodec     59. 54.100 / 59. 54.100
  libavformat    59. 34.102 / 59. 34.102
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 51.100 /  8. 51.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
[avi @ 0x5653547dd400] Could not find codec parameters for stream 0 (Video: none (SLES / 0x53454C53), none, 320x240, 14132 kb/s): unknown codec
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, avi, from 'SoftLab-NSK_advanced_I-frames_2.avi':
  Duration: 00:00:01.13, start: 0.000000, bitrate: 13672 kb/s
  Stream #0:0: Video: none (SLES / 0x53454C53), none, 320x240, 14132 kb/s, 23.97 fps, 23.97 tbr, 23.97 tbn
At least one output file must be specified