graphics/png: Update to 1.6.57

Fixes CVE-2026-34757

Changelog: https://github.com/pnggroup/libpng/blob/v1.6.57/ANNOUNCE

PR: 294349
Reported by: Christos Chatzaras <chris@cretaforce.gr>
Reviewed by: desktop (arrowd) (previous iteration)

(cherry picked from commit fd1980fafbbede3b50816970488013516da84394)