iff ilbm: add ehb color mode

[ami_stuff]

​http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode

SDL_image supports this mode.

​http://www.libsdl.org/projects/SDL_image/

IMG_lbm.c:

		/* Amiga EHB mode (Extra-Half-Bright) */
		/* 6 bitplanes mode with a 32 colors palette */
		/* The 32 last colors are the same but divided by 2 */
		/* Some Amiga pictures save 64 colors with 32 last wrong colors, */
		/* they shouldn't !, and here we overwrite these 32 bad colors. */
		if ( (nbcolors==32 || flagEHB ) && (1<<bmhd.planes)==64 )
		{
			nbcolors = 64;
			ptr = &colormap[0];
			for ( i=32; i<64; i++ )
			{
				Image->format->palette->colors[i].r = (*ptr++)/2;
				Image->format->palette->colors[i].g = (*ptr++)/2;
				Image->format->palette->colors[i].b = (*ptr++)/2;
			}
		}

C:\>ffmpeg -i KING.IFF
ffmpeg version N-34704-g985e768, Copyright (c) 2000-2011 the FFmpeg developers
  built on Nov 11 2011 01:49:15 with gcc 4.6.2
  configuration: --disable-static --enable-shared --enable-gpl --enable-version3
 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzli
b --enable-frei0r --enable-libopencore-amrnb --enable-libopencore-amrwb --enable
-libfreetype --enable-libgsm --enable-libmp3lame --enable-libopenjpeg --enable-l
ibrtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-li
bvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-l
ibx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil    51. 24. 1 / 51. 24. 1
  libavcodec   53. 31. 0 / 53. 31. 0
  libavformat  53. 20. 0 / 53. 20. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 47. 2 /  2. 47. 2
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[iff_byterun1 @ 003EA360] ExtraHalfBrite (EHB) mode not supported
    Last message repeated 1 times
[IFF @ 003E9B00] Estimating duration from bitrate, this may be inaccurate
Input #0, IFF, from 'KING.IFF':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: iff_byterun1 (ILBM / 0x4D424C49), pal8, 320x256, SAR 44:
44 DAR 5:4, 90k tbr, 90k tbn, 90k tbc
At least one output file must be specified

[ami_stuff]

This file is better for test.

[Carl Eugen Hoyos]

Implemented by Peter Ross.