Remuxing from flv to swf breaks framerate

[Carl Eugen Hoyos]

(As reported on the forum.)
Remuxing from flv to swf produces output files with 232fps that play as fast as the system allows.

$ ffmpeg -i fate-suite/flash-vp6/clip1024.flv -vcodec copy out.swf
ffmpeg version N-62025-g909757f Copyright (c) 2000-2014 the FFmpeg developers
  built on Apr  1 2014 22:56:40 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 71.100 / 52. 71.100
  libavcodec     55. 56.107 / 55. 56.107
  libavformat    55. 36.100 / 55. 36.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, flv, from 'fate-suite/flash-vp6/clip1024.flv':
  Metadata:
    audiodelay      : 0
    creationdate    : Fri Sep 02 16:45:11 2005
                    :
  Duration: 00:00:17.30, start: 0.000000, bitrate: 51 kb/s
    Stream #0:0: Video: vp6f, yuv420p, 112x80, 49 kb/s, 10 tbr, 1k tbn, 1k tbc
Output #0, swf, to 'out.swf':
  Metadata:
    audiodelay      : 0
    creationdate    : Fri Sep 02 16:45:11 2005
                    :
    encoder         : Lavf55.36.100
    Stream #0:0: Video: vp6f, yuv420p, 112x80, q=2-31, 49 kb/s, 90k tbn, 1k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame=  174 fps=0.0 q=-1.0 Lsize=     109kB time=00:00:17.30 bitrate=  51.7kbits/s
video:106kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 3.095598%
$ ffmpeg -i out.swf
ffmpeg version N-62025-g909757f Copyright (c) 2000-2014 the FFmpeg developers
  built on Apr  1 2014 22:56:40 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 71.100 / 52. 71.100
  libavcodec     55. 56.107 / 55. 56.107
  libavformat    55. 36.100 / 55. 36.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, swf, from 'out.swf':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: vp6f, yuv420p, 112x80, 232 fps, 232 tbr, 232 tbn
At least one output file must be specified

$ make fate-lavf-flv_fmt
$ ffmpeg -i tests/data/lavf/lavf.flv -vcodec copy out2.swf
ffmpeg version N-62025-g909757f Copyright (c) 2000-2014 the FFmpeg developers
  built on Apr  1 2014 22:56:40 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 71.100 / 52. 71.100
  libavcodec     55. 56.107 / 55. 56.107
  libavformat    55. 36.100 / 55. 36.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, flv, from 'tests/data/lavf/lavf.flv':
  Metadata:
    title           : lavftest
  Duration: 00:00:01.00, start: 0.000000, bitrate: 2636 kb/s
    Stream #0:0: Video: flv1, yuv420p, 352x288, 200 kb/s, 25 tbr, 1k tbn, 1k tbc
Output #0, swf, to 'out2.swf':
  Metadata:
    title           : lavftest
    encoder         : Lavf55.36.100
    Stream #0:0: Video: flv1, yuv420p, 352x288, q=2-31, 200 kb/s, 90k tbn, 1k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame=   25 fps=0.0 q=-1.0 Lsize=     322kB time=00:00:00.96 bitrate=2742.8kbits/s
video:321kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.159599%
$ ffmpeg -i out2.swf
ffmpeg version N-62025-g909757f Copyright (c) 2000-2014 the FFmpeg developers
  built on Apr  1 2014 22:56:40 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 71.100 / 52. 71.100
  libavcodec     55. 56.107 / 55. 56.107
  libavformat    55. 36.100 / 55. 36.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, swf, from 'out2.swf':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: flv1, yuv420p, 352x288, 232 fps, 232 tbr, 232 tbn
At least one output file must be specified