jpeg gray 12bpp: use gray12 pix_fmt for decoding 12bpp gray files

[ami_stuff]

lossy:

​https://trac.ffmpeg.org/raw-attachment/ticket/4683/12bpp.jpg

lossless:

​https://trac.ffmpeg.org/raw-attachment/ticket/855/12_line.jpg

also currently -vlowres does not work correctly with 12bpp gray lossy files

./ffmpeg -i 12bpp.jpg -y out.bmp
ffmpeg version N-112362-g4c422de1db 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. 30.101 / 60. 30.101
  libavformat    60. 15.100 / 60. 15.100
  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, image2, from '12bpp.jpg':
  Duration: 00:00:00.04, start: 0.000000, bitrate: 65308 kb/s
  Stream #0:0: Video: mjpeg (Sequential), gray16le(12 bpc, bt470bg/unknown/unknown), 999x749 [SAR 300:300 DAR 999:749], 25 fps, 25 tbr, 25 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> bmp (native))
Press [q] to stop, [?] for help
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf60.15.100
  Stream #0:0: Video: bmp, gray(pc, bt470bg/unknown/unknown, progressive), 999x749 [SAR 1:1 DAR 999:749], q=2-31, 200 kb/s, 25 fps, 25 tbn
    Metadata:
      encoder         : Lavc60.30.101 bmp
frame=    0 fps=0.0 q=-0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=N/[image2 @ 0x55947d882f80] The specified filename 'out.bmp' does not contain an image sequence pattern or a pattern is invalid.
[image2 @ 0x55947d882f80] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
[out#0/image2 @ 0x55947d882e40] video:732kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed=   0x