Colour space auto-selection does not work well for rgb48 input and yuv output

[Carl Eugen Hoyos]

Related to:
​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/37525

When encoding rgb48 with an encoder (like ffv1) that supports yuv@16bit but not rgb@16bit, yuv420p16 is auto-selected instead of yuv444p16.

$ ffmpeg -i tests/lena.pnm -pix_fmt rgb48 out.png
...
$ ffmpeg -i out.png -vcodec ffv1 -coder 1 out.avi
ffmpeg version N-41334-g647e2e0 Copyright (c) 2000-2012 the FFmpeg developers
  built on Jun  7 2012 09:18:33 with gcc 4.3.2
  configuration: --cc=/usr/local/gcc-4.3.2/bin/gcc --enable-gpl --enable-libopenjpeg --enable-libvorbis --enable-libspeex --enable-libmp3lame --enable-libtheora --extra-ldflags=-lm --enable-libvpx --enable-libxavs
  libavutil      51. 56.100 / 51. 56.100
  libavcodec     54. 24.100 / 54. 24.100
  libavformat    54.  6.101 / 54.  6.101
  libavdevice    54.  0.100 / 54.  0.100
  libavfilter     2. 78.100 /  2. 78.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, image2, from 'out.png':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, rgb48be, 256x256, 25 tbr, 25 tbn, 25 tbc
[buffer @ 0x9012040] w:256 h:256 pixfmt:rgb48be tb:1/25 sar:0/1 sws_param:flags=2
[ffmpeg_buffersink @ 0x9011fa0] No opaque field provided
[format @ 0x9012500] auto-inserting filter 'auto-inserted scaler 0' between the filter 'Parsed_null_0' and the filter 'format'
[auto-inserted scaler 0 @ 0x9012420] w:256 h:256 fmt:rgb48be sar:0/1 -> w:256 h:256 fmt:yuv420p16le sar:0/1 flags:0x4
stream->bits_per_coded_sample: 0, stream->extradata_size: 0
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf54.6.101
    Stream #0:0: Video: ffv1 (FFV1 / 0x31564646), yuv420p16le, 256x256, q=2-31, 200 kb/s, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (png -> ffv1)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=     140kB time=00:00:00.04 bitrate=28654.4kbits/s
video:134kB audio:0kB global headers:0kB muxing overhead 4.147827%

[Michael Niedermayer]

FIxed by the ticket1517 fix