IFF ANIM: support anims with sound

[ami_stuff]

more samples are here:

​https://www.datafilehost.com/d/ffed7001

C:\>ffmpeg -i EntWarp.sndanim out.avi
ffmpeg version 3.0.git Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-pthreads --enable-gpl --disable-ffprobe
  libavutil      55. 24.100 / 55. 24.100
  libavcodec     57. 40.100 / 57. 40.100
  libavformat    57. 36.100 / 57. 36.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 45.100 /  6. 45.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[iff @ 021ad1a0] Warning: not compiled with thread support, using thread emulati
on
Input #0, iff, from 'EntWarp.sndanim':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: iff_ilbm (ANIM / 0x4D494E41), rgb0, 352x282, 20 fps, 60
tbr, 60 tbn
[iff @ 02538c00] Warning: not compiled with thread support, using thread emulati
on
[mpeg4 @ 025af5c0] Warning: not compiled with thread support, using thread emula
tion
[avi @ 025380e0] Using AVStream.codec to pass codec parameters to muxers is depr
ecated, use AVStream.codecpar instead.
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf57.36.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 352x282, q=2-31, 200
 kb/s, 60 fps, 60 tbn
    Metadata:
      encoder         : Lavc57.40.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
Stream mapping:
  Stream #0:0 -> #0:0 (iff_ilbm (iff) -> mpeg4 (native))
Press [q] to stop, [?] for help
[mpeg4 @ 025af5c0] ignoring invalid SAR: 0/0
    Last message repeated 50 times
frame=   51 fps=0.0 q=8.3 Lsize=      57kB time=00:00:02.71 bitrate= 172.7kbits/
s speed=34.8x
video:48kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing ov
erhead: 19.627884%

[MaddTheSane]

One difficulty with implementing ANIM sound support is that in many files the samples used for sound might be at different sampling rates. Also, some of them might play more than one sample at once.

Looking at entwarp.anim in MultiShow, it seems to use a different format than what I've seen in other ANIMs.

More samples I've uploaded can be found on ticket #6175