Support PLII upmixing

[Carl Eugen Hoyos]

​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/54545

FFmpeg does support PLII down-mixing (see tickets #709 and #3455) but does not support upmixing afaict. I will attach a sample from the MythTV collection that plays surround sound on my receiver, upmixing to six channels does not produce a similar result.

$ ffmpeg -i channelsPLII.mpg -ac 6 out.wav
ffmpeg version N-67396-g90cdec5 Copyright (c) 2000-2014 the FFmpeg developers
  built on Nov  6 2014 00:16:59 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 11.100 / 54. 11.100
  libavcodec     56. 10.102 / 56. 10.102
  libavformat    56. 12.101 / 56. 12.101
  libavdevice    56.  2.100 / 56.  2.100
  libavfilter     5.  2.103 /  5.  2.103
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, mpeg, from 'channelsPLII.mpg':
  Duration: 00:00:31.82, start: 0.740000, bitrate: 462 kb/s
    Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p(tv), 480x576 [SAR 8:5 DAR 4:3], max. 2420 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
    Stream #0:1[0x1c0]: Audio: mp2, 48000 Hz, stereo, s16p, 224 kb/s
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf56.12.101
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 5.1, s16, 4608 kb/s
    Metadata:
      encoder         : Lavc56.10.102 pcm_s16le
Stream mapping:
  Stream #0:1 -> #0:0 (mp2 (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
size=   17915kB time=00:00:31.84 bitrate=4608.0kbits/s
video:0kB audio:17914kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000556%

MythTV and VLC contain PLII decoder that support upmixing under GPL.