graphics/png: Update to 1.6.45

Changelog: https://github.com/pnggroup/libpng/blob/v1.6.45/ANNOUNCE

PR: 284007
Reviewed by: desktop (arrowd)
Sponsored by: Blinkinblox
Exp-run by: antoine

(cherry picked from commit 5b7d14496a1cb41ed8b337f0a142d74527a06c04)