png: update to 1.6.36.

Version 1.6.36 [December 1, 2018]
  Optimized png_do_expand_palette for ARM processors.
  Improved performance by around 10-22% on a recent ARM Chromebook.
  (Contributed by Richard Townsend, ARM Holdings)
  Fixed manipulation of machine-specific optimization options.
  (Contributed by Vicki Pfau)
  Used memcpy instead of manual pointer arithmetic on Intel SSE2.
  (Contributed by Samuel Williams)
  Fixed build errors with MSVC on ARM64.
  (Contributed by Zhijie Liang)
  Fixed detection of libm in CMakeLists.
  (Contributed by Cameron Cawley)
  Fixed incorrect creation of pkg-config file in CMakeLists.
  (Contributed by Kyle Bentley)
  Fixed the CMake build on Windows MSYS by avoiding symlinks.
  Fixed a build warning on OpenBSD.
  (Contributed by Theo Buehler)
  Fixed various typos in comments.
  (Contributed by "luz.paz")
  Raised the minimum required CMake version from 3.0.2 to 3.1.
  Removed yet more of the vestigial support for pre-ANSI C compilers.
  Removed ancient makefiles for ancient systems that have been broken
  across all previous libpng-1.6.x versions.
  Removed the Y2K compliance statement and the export control
  information.
  Applied various code style and documentation fixes.

(wiz)