support image2pipe input without parser

[cehoyos]

(issue 2683)

image2pipe fails for codecs that do not have a parser.

$ ./ffmpeg -i tests/lena.pnm out.bmp

...

$ cat out.bmp |./ffmpeg -f image2pipe -vcodec bmp -i -
ffmpeg version N-40924-g349c624 Copyright (c) 2000-2012 the FFmpeg developers
  built on May 21 2012 07:59:42 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
  libavutil      51. 53.100 / 51. 53.100
  libavcodec     54. 21.101 / 54. 21.101
  libavformat    54.  5.101 / 54.  5.101
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 75.100 /  2. 75.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
[bmp @ 0x8f1ac20] not enough data (4096 < 196662), trying to decode anyway
[bmp @ 0x8f1ac20] not enough data (4042 < 196608)
[image2pipe @ 0x8f0c4a0] Estimating duration from bitrate, this may be inaccurate
Input #0, image2pipe, from 'pipe:':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: bmp, bgr24, 256x256, 25 fps, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[cehoyos]

The bmp parser was implemented by Paul B Mahol.

[cehoyos]

Parsers are still missing for at least dpx, jpegls, gif, pcx, sgi, sunrast, targa and tiff.