libpng: security bump to version 1.6.20

Fixes:
CVE-2015-8126 - incorrect implementation of png_set_PLTE() that uses
png_ptr not info_ptr, that left png_set_PLTE() open to this vuln.

(fix in previous release was incomplete)

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>