Log Message:
Updated png to 1.6.28.

Version 1.6.28rc01 [January 3, 2017]
  Fixed arm/aarch64 detection in CMakeLists.txt (Gianfranco Costamagna).
  Added option to Cmake build allowing a custom location of zlib to be
    specified in a scenario where libpng is being built as a subproject
    alongside zlib by another project (Sam Serrels).
  Changed png_ptr->options from a png_byte to png_uint_32, to accomodate
    up to 16 options.

Version 1.6.28rc02 [January 4, 2017]
  Added "include(GNUInstallDirs)" to CMakeLists.txt (Gianfranco \ 
Costamagna).
  Moved SSE2 optimization code into the main libpng source directory.
    Configure libpng with "configure --enable-intel-sse" or compile
    libpng with "-DPNG_INTEL_SSE" in CPPFLAGS to enable it.

Version 1.6.28rc03 [January 4, 2017]
  Backed out the SSE optimization and last CMakeLists.txt to allow time for QA.

Version 1.6.28 [January 5, 2017]
  No changes.