Support converting to GBRP

[Carl Eugen Hoyos]

The dpx encoder supports different GBRP colour-spaces, but the scaler cannot convert into GBRP.

$ ffmpeg -i tests/lena.pnm -pix_fmt gbrp10 out.dpx
ffmpeg version N-44649-g6d87781 Copyright (c) 2000-2012 the FFmpeg developers
  built on Sep 20 2012 22:52:22 with gcc 4.5.3 (GCC)
  configuration: --cc=/usr/local/gcc-4.5.3/bin/gcc --enable-gpl
  libavutil      51. 73.101 / 51. 73.101
  libavcodec     54. 56.100 / 54. 56.100
  libavformat    54. 27.101 / 54. 27.101
  libavdevice    54.  2.101 / 54.  2.101
  libavfilter     3. 16.106 /  3. 16.106
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 tbr, 25 tbn, 25 tbc
Impossible to convert between the formats supported by the filter 'Parsed_null_0' and the filter 'auto-inserted scaler 0'
Error opening filters!

[Carl Eugen Hoyos]

Implemented by Michael.