This is what gimp, ImageMagick and FreeImage do and what the
Adobe Photoshop file format specification suggests.
Fixes a sample from ticket #6045.
Reviewed-by: Martin Vignali
Decode the Image Data Section (which contains merged pictures).
Support RGB/A and Grayscale/A in 8bits and 16 bits per channel.
Support uncompress and rle decompression in Image Data Section.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>