smc enc: artefacts in the output file

[ami_stuff]

other issue: ffplay plays "gold2.avi" file incorrectly (only mouse pointer is visible)

./ffmpeg -i gold2.avi -vcodec smc -y out.mov
ffmpeg version N-109101-g822da7a317 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: --enable-libopenjpeg
  libavutil      57. 42.100 / 57. 42.100
  libavcodec     59. 52.102 / 59. 52.102
  libavformat    59. 34.101 / 59. 34.101
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 50.100 /  8. 50.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
Input #0, avi, from 'gold2.avi':
  Duration: 00:00:12.00, start: 0.000000, bitrate: 153 kb/s
  Stream #0:0: Video: srgc (srgc / 0x63677273), bgra, 299x171, 149 kb/s, 15 fps, 15 tbr, 15 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (srgc (native) -> smc (native))
Press [q] to stop, [?] for help
Output #0, mov, to 'out.mov':
  Metadata:
    encoder         : Lavf59.34.101
  Stream #0:0: Video: smc (smc  / 0x20636D73), pal8(pc, progressive), 299x171, q=2-31, 200 kb/s, 15 fps, 15360 tbn
    Metadata:
      encoder         : Lavc59.52.102 smc
frame=    0 fps=0.0 q=-0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=  frame=  180 fps=0.0 q=-0.0 Lsize=     469kB time=00:00:11.93 bitrate= 322.1kbits/s speed=41.1x    
video:466kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.750305%

./ffmpeg -i out.mov -vcodec rawvideo -y out2.avi
ffmpeg version N-109101-g822da7a317 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: --enable-libopenjpeg
  libavutil      57. 42.100 / 57. 42.100
  libavcodec     59. 52.102 / 59. 52.102
  libavformat    59. 34.101 / 59. 34.101
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 50.100 /  8. 50.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 512
    compatible_brands: qt  
    encoder         : Lavf59.34.101
  Duration: 00:00:12.00, start: 0.000000, bitrate: 320 kb/s
  Stream #0:0[0x1]: Video: smc (smc  / 0x20636D73), pal8(progressive), 299x171, 317 kb/s, 15 fps, 15 tbr, 15360 tbn (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : FFMP
      encoder         : Lavc59.52.102 smc
Stream mapping:
  Stream #0:0 -> #0:0 (smc (native) -> rawvideo (native))
Press [q] to stop, [?] for help
Output #0, avi, to 'out2.avi':
  Metadata:
    major_brand     : qt  
    minor_version   : 512
    compatible_brands: qt  
    ISFT            : Lavf59.34.101
  Stream #0:0: Video: rawvideo, pal8(progressive), 299x171, q=2-31, 6135 kb/s, 15 fps, 15 tbn (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : FFMP
      encoder         : Lavc59.52.102 rawvideo
frame=    0 fps=0.0 q=0.0 size=       0kB time=-577014:32:22.77 bitrate=  -0.0kbframe=  180 fps=0.0 q=-0.0 Lsize=    9028kB time=00:00:11.93 bitrate=6197.8kbits/s speed= 338x    
video:9168kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[Elon Musk]

Fixed artifacts when encoding with this input.

Note that video have alpha, so encoding is fine when only mouse is "visible".