Support for GMP4 variant of MPEG-4 ASP

[cehoyos]

Attached file shows I-Frames with mplayer -vc +ffodivx and attached riff patch. Other frame types only output "header damaged".

$ ffmpeg -v 9 -loglevel 99 -i gmp4.avi out.avi
ffmpeg version N-32325-g6f8b1fc, Copyright (c) 2000-2011 the FFmpeg developers
  built on Sep  5 2011 10:13:11 with gcc 4.5.3
  configuration: --cc=/usr/local/gcc-4.5.3/bin/gcc
  libavutil    51. 16. 0 / 51. 16. 0
  libavcodec   53. 13. 0 / 53. 13. 0
  libavformat  53. 11. 0 / 53. 11. 0
  libavdevice  53.  3. 0 / 53.  3. 0
  libavfilter   2. 37. 0 /  2. 37. 0
  libswscale    2.  1. 0 /  2.  1. 0
[avi @ 0x129f780] Format avi probed with size=2048 and score=100
[AVI demuxer @ 0x129fda0] use odml:1
[avi @ 0x129f780] Something went wrong during header parsing, I will ignore it and try to continue anyway.
[mpeg4 @ 0x12a17a0] hmm, seems the headers are not complete, trying to guess time_increment_bits
[mpeg4 @ 0x12a17a0] my guess is 5 bits ;)
[avi @ 0x129f780] probing stream 1 pp:2500

...

[avi @ 0x129f780] probing stream 1 pp:1443
[avi @ 0x129f780] probed stream 1 failed
[avi @ 0x129f780] max_analyze_duration 5000000 reached at 5000000
Input #0, avi, from 'gmp4.avi':
  Duration: 00:05:00.30, start: 0.000000, bitrate: 68 kb/s
    Stream #0.0, 152, 1/30: Video: mpeg4 (GMP4 / 0x34504D47), yuv420p, 320x240, 1/30, 30 fps, 30 tbr, 30 tbn, 30 tbc
    Stream #0.1, 152, 1/30: Subtitle: none
[buffer @ 0x15b2300] w:320 h:240 pixfmt:yuv420p tb:1/1000000 sar:0/1 sws_param:
[mpeg4 @ 0x12a2920] Unsupported bit depth: 0
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf53.11.0
    Stream #0.0, 0, 1/30: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 320x240, 1/30, q=2-31, 200 kb/s, 30 tbn, 30 tbc
Stream mapping:
  Stream #0.0 -> #0.0 (mpeg4 -> mpeg4)
Press [q] to stop, [?] for help
[mpeg4 @ 0x12a17a0] Unsupported bit depth: 0
[mpeg4 @ 0x12a17a0] hmm, seems the headers are not complete, trying to guess time_increment_bits
[mpeg4 @ 0x12a17a0] my guess is 5 bits ;)
[mpeg4 @ 0x12a17a0] looks like this file was encoded with (divx4/(old)xvid/opendivx) -> forcing low_delay flag
[mpeg4 @ 0x12a17a0] header damaged
Error while decoding stream #0.0

...

Error while decoding stream #0.0
    Last message repeated 1054 times
frame=    9 fps=  0 q=10.8 Lsize=     148kB time=00:00:32.03 bitrate=  37.8kbits/s    ^M
video:120kB audio:0kB global headers:0kB muxing overhead 23.445171%