contrib: png: update to 1.6.37

From the release notes of 1.6.36:
"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)."

1.6.37:
This release fixed CVE-2019-7317, although our png module is not using this API.

(cherry picked from commit a643b2fa)
(cherry picked from commit 6bfeb1d8)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>