Changes only pngvalid and test-pngvalid-full.sh - no libpng changes -
 corrects the code for palette image tests, disables attempts to validate
 palette colors.

 This problem is present in 1.5.2; if png_set_background is called with
 need_expand false and the matching 16 bit color libpng erroneously just treats
 it as an 8-bit color because of where png_do_expand_16 is in the transform
 list.  This simple fix reduces the supplied colour to 8 bits, so it gets
 smashed, but this is better than the current behavior.

png_expand_16 back where it was before libpng-1.5.3beta07.
The change doesn't work because it requires 16 bit gamma tables when the code
only generates 8 bit ones.  This fails silently; the libpng code just doesn't
do any gamma correction.  Moving the tests back leaves the old, inaccurate, 8
bit gamma calculations, but these are clearly better than none!

also minor cleanup in the libpng code itself (pngrtran.c and pngrutil.c) and some
extra checking there.

  libpng-manual.txt.
  The cHRM chunk now sets the defaults for png_set_rgb_to_gray() (when negative
    parameters are supplied by the caller), while in the absence of cHRM
    sRGB/Rec 709 values are still used.
  The bKGD chunk no longer overwrites the background value set by
    png_set_background(), allowing the latter to be used before the file
    header is read. It never performed any useful function to override
    the default anyway.

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before IDAT.

PNG_AFTER_IDAT from pngpriv.h to png.h because they must be visible to
applications that call png_set_unknown_chunks().

minor change from the previous (unreleased) ABI/API to hide the exact value
used for Macs - it's not a good idea to embed this in the ABI!

that this only affects how composited pixels are returned to the calling
application; they don't affect the pixels that are written to a PNG file.

some currently dead code in pngrtran.c that will be enabled in libpng-1.6

 (libpng-manual.txt still to be updated, see png.h for documentation.)