Update to libpng 1.5.26

Created by: akien-mga

Includes numerous security fixes compared to the previous version which was around 1.5.7 or so. Fixes #2787 (closed)

This was just a "drop new source files and go", and then Godot still seemed to build fine on X11 and run my projects properly. Might need some testing on other platforms first, though it's still the 1.5.x branch so I don't expect major breakages.