Log Message:
Updated png to 1.6.26.

Version 1.6.26beta01 [September 26, 2016]
  Fixed handling zero length IDAT in pngfix (bug report by Agostino Sarubbo,
    bugfix by John Bowler).
  Do not issue a png_error() on read in png_set_pCAL() because png_handle_pCAL
    has allocated memory that libpng needs to free.
  Conditionally compile png_set_benign_errors() in pngread.c and pngtest.c
  Issue a png_benign_error instead of a png_error on ADLER32 mismatch
    while decoding compressed data chunks.
  Changed PNG_ZLIB_VERNUM to ZLIB_VERNUM in pngpriv.h, pngstruct.h, and
    pngrutil.c.
  If CRC handling of critical chunks has been set to PNG_CRC_QUIET_USE,
    ignore the ADLER32 checksum in the IDAT chunk as well as the chunk CRCs.
  Issue png_benign_error() on ADLER32 checksum mismatch instead of png_error().
  Add tests/badcrc.png and tests/badadler.png to tests/pngtest.
  Merged pngtest.c with libpng-1.7.0beta84/pngtest.c

Version 1.6.26beta02 [October 1, 2016]
  Updated the documentation about CRC and ADLER32 handling.
  Quieted 117 warnings from clang-3.8 in pngtrans.c, pngread.c,
     pngwrite.c, pngunknown.c, and pngvalid.c.
  Quieted 58 (out of 144) -Wconversion compiler warnings by changing
    flag definitions in pngpriv.h from 0xnnnn to 0xnnnnU and trivial changes
    in png.c, pngread.c, and pngwutil.c.

Version 1.6.26beta03 [October 2, 2016]
  Removed contrib/libtests/*.orig and *.rej that slipped into the tarballs.
  Quieted the 86 remaining -Wconversion compiler warnings by
    revising the png_isaligned() macro and trivial changes in png.c,
    pngerror.c, pngget.c, pngmem.c, pngset.c, pngrtran.c, pngrutil.c,
    pngwtran.c, pngwrite.c, and pngwutil.c.

Version 1.6.26beta04 [October 3, 2016]
  Quieted (bogus?) clang warnings about "absolute value has no effect"
    when PNG_USE_ABS is defined.
  Fixed offsets in contrib/intel/intel_sse.patch

Version 1.6.26beta05 [October 6, 2016]
  Changed integer constant 4294967294 to unsigned 4294967294U in pngconf.h
    to avoid a signed/unsigned compare in the preprocessor.

Version 1.6.26beta06 [October 7, 2016]
  Use zlib-1.2.8.1 inflateValidate() instead of inflateReset2() to
    optionally avoid ADLER32 evaluation.

Version 1.6.26rc01 [October 12, 2016]
  No changes.

Version 1.6.26 [October 20, 2016]
  Cosmetic change, "ptr != 0" to "ptr != NULL" in png.c and \ 
pngrutil.c
  Despammed email addresses (replaced "@" with " at ").