scpr: fix 16-bit decoding

[ami_stuff]

./ffmpeg -i scpr_v3_16bit.avi -y out.avi
ffmpeg version N-112239-g5ba06ad0b8 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 11 (Ubuntu 11.4.0-1ubuntu1~22.04)
  configuration: 
  libavutil      58. 27.100 / 58. 27.100
  libavcodec     60. 27.100 / 60. 27.100
  libavformat    60. 13.100 / 60. 13.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9. 11.100 /  9. 11.100
  libswscale      7.  4.100 /  7.  4.100
  libswresample   4. 11.100 /  4. 11.100
Input #0, avi, from 'scpr_v3_16bit.avi':
  Duration: 00:00:03.80, start: 0.000000, bitrate: 324 kb/s
  Stream #0:0: Video: scpr (SCPR / 0x52504353), rgb0, 272x198, 322 kb/s, 15 fps, 15 tbr, 15 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (scpr (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
[vist#0:0/scpr @ 0x55f3a82bdb40] Error submitting packet to decoder: Invalid data found when processing input
    Last message repeated 56 times
[vost#0:0/mpeg4 @ 0x55f3a82bf280] No filtered frames for output stream, trying to initialize anyway.
[mpeg4 @ 0x55f3a82bf740] too many threads/slices (14), reducing to 13
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf60.13.100
  Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p(progressive), 272x198, q=2-31, 200 kb/s, 15 tbn
    Metadata:
      encoder         : Lavc60.27.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A
[vist#0:0/scpr @ 0x55f3a82bdb40] Decode error rate 1 exceeds maximum 0.666667
[out#0/avi @ 0x55f3a82bc3c0] video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
[out#0/avi @ 0x55f3a82bc3c0] Output file is empty, nothing was encoded(check -ss / -t / -frames parameters if used)
frame=    0 fps=0.0 q=0.0 Lsize=       6kB time=N/A bitrate=N/A speed=N/A    
Conversion failed!

[Elon Musk]

Does windows one decodes it fine? because v3 is known to have buggy implementation of encoder, thus v4 version of codec come around.

[ami_stuff]

Yes, attached file decodes correctly with the windows v3 decoder.