graphics/png: Update to 1.6.56

Fixes CVE-2026-33416 and CVE-2026-33636

Initial patch submitted by Fabian Wenk

Changelog: https://github.com/pnggroup/libpng/blob/v1.6.56/ANNOUNCE

PR: 294061
Reviewed by: ports-secteam (fluffy)
Exp-run by: antoine

(cherry picked from commit 9daa967c64e010af5caccef10e97113dffabdab2)