contrib: png: update to 1.6.36

From the release notes:

"Fixes some build issues, adds a couple of small optimizations (ARM
png_do_expand_palette(), Intel SSE2 memcpy()), and updates the license
(identical terms to the zlib license, with the old license appended in the
manner of the Python Software Foundation License version 2, and the list of
contributing authors moved to a separate AUTHORS file)."

contrib/src/png/SHA512SUMS

@@ -1 +1 @@
-b69152bc7869e98536effa8e45e2dac7bffee26aa9e31fc90b37506a702256919bcf4dd9cadc42fa4e45143bc023ffd7c637742367e58707e8c2226b3a09eee0  libpng-1.6.35.tar.xz
+a86ee977df69748e5039fb0ead883f1d3f88b8a701fa24cf8e62dd77c5871bb46397d794fa33ec1d0be1ac488246832ad79d0e6117ac093bdce1b2a1cfcb2bb0  libpng-1.6.36.tar.xz

contrib/src/png/rules.mak

@@ -1,5 +1,5 @@
 # PNG
-PNG_VERSION := 1.6.35
+PNG_VERSION := 1.6.36
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png