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.)

This was because the attempt to reset the zlib stream in png_write_IDAT
happened after the first IDAT chunk had been deflated - much too late.
In this change internal functions are added to claim/release the z_stream
and, hopefully, make the code more robust.  Also deflateEnd checking is
added - previously libpng would ignore an error at the end of the stream.

optimization configureable.

 This change adds internal APIs to allow png_warning messages to have parameters
 without requiring the host OS to implelment snprintf.  As a side effect the
 dependency of the RFC1132 code on stdio is removed and PNG_NO_WARNINGS does
 actually work now.

to ensure libpng sees the correct API.

and png_info definitions.

structure in the libpng documentation.

handling. Internal changes only except for a new option to avoid compiling
in the png_build_grayscale_palette API (which is not used at all internally.)
The main change is to move the transform tests (READ_TRANSFORMS,
WRITE_TRANSFORMS) up one level to the caller of the APIs.