png_set_scale_16_to_8 from pngpriv.h

    1.5.3beta06
  Also removed a spurious (totally unused and confusing) member from png_info.
  The palette expand optimization prevented expansion to an intermediate RGBA
    form if tRNS was present but alpha was marked to be stripped; this exposed
    a check for tRNS in png_do_expand_palette() which is inconsistent with the
    code elsewhere in libpng.

by disabling PNG_CHOP_16_TO_8_SUPPORTED

called.

Fixed some compiliation problems with scaling options.

to get the same (inaccurate) output as libpng-1.5.2 and earlier.

    wrong (high by one) 25% of the time. Dividing by 257 with rounding is
    wrong in 128 out of 65536 cases. Getting the right answer all the time
    without division is easy.

 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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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!

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 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.

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.