Log Message:
Updated png to 1.6.27, security fix release.

Version 1.6.27beta01 [November 2, 2016]
  Restrict the new ADLER32-skipping to IDAT chunks.  It broke iCCP chunk
    handling: an erroneous iCCP chunk would throw a png_error and reject the
    entire PNG image instead of rejecting just the iCCP chunk with a warning,
    if built with zlib-1.2.8.1.

Version 1.6.27rc01 [December 27, 2016]
  Control ADLER32 checking with new PNG_IGNORE_ADLER32 option.
  Removed the use of a macro containing the pre-processor 'defined'
    operator.  It is unclear whether this is valid; a macro that
    "generates" 'defined' is not permitted, but the use of the word
    "generates" within the C90 standard seems to imply more than simple
    substitution of an expression itself containing a well-formed defined
    operation.
  Added ARM support to CMakeLists.txt (Andreas Franek).

Version 1.6.27 [December 29, 2016]
  Fixed a potential null pointer dereference in png_set_text_2() (bug report
    and patch by Patrick Keshishian).