Log Message:
Update png to 1.6.21:

Version 1.6.21beta01 [December 11, 2015]
  Fixed syntax "$(command)" in tests/pngstest that some shells other than
    bash could not parse (Bug report by Nelson Beebe). Use `command` instead.

Version 1.6.21beta02 [December 14, 2015]
  Moved png_check_keyword() from pngwutil.c to pngset.c
  Removed LE/BE dependencies in pngvalid, to 'fix' the current problem
    in the BigEndian tests by not testing it, making the BE code the same
    as the LE version.
  Fixes to pngvalid for various reduced build configurations (eliminate unused
    statics) and a fix for the case in rgb_to_gray when the digitize option
    reduces graylo to 0, producing a large error.

Version 1.6.21beta03 [December 18, 2015]
  Widened the 'limit' check on the internally calculated error limits in
    the 'DIGITIZE' case (the code used prior to 1.7 for rgb_to_gray error
    checks) and changed the check to only operate in non-release builds
    (base build type not RC or RELEASE.)
  Fixed undefined behavior in pngvalid.c, undefined because
    (png_byte) << shift is undefined if it changes the signed bit
    (because png_byte is promoted to int). The libpng exported functions
    png_get_uint_32 and png_get_uint_16 handle this. (Bug reported by
    David Drysdale as a result of reports from UBSAN in clang 3.8).
  This changes pngvalid to use BE random numbers; this used to produce
    errors but these should not be fixed as a result of the previous changes.

Version 1.6.21rc01 [January 4, 2016]
  In projects/vstudio, combined readme.txt and WARNING into README.txt

Version 1.6.21rc02 [January 7, 2016]
  Relocated assert() in contrib/tools/pngfix.c, bug found by American
    Fuzzy Lop, reported by Brian Carpenter.
  Marked 'limit' UNUSED in transform_range_check().  This only affects
    release builds.

Version 1.6.21 [January 15, 2016]
  Worked around a false-positive Coverity issue in pngvalid.c.