Update libpng.py to 1.6.56

[HCL-JasonR]

This fixes two High CVEs.
https://www.cve.org/CVERecord?id=CVE-2026-33416
https://www.cve.org/CVERecord?id=CVE-2026-33636

[sbc100]

Done!

[HCL-JasonR]

Thanks! I'll write up a ChangeLog entry and remove this comment.

[sbc100]

Maybe add a ChangeLot entry?

[dasa]

@HCL-JasonR thanks for creating this PR! Can you try bringing your branch up to date with main? Maybe that'll fix the tests.

[HCL-JasonR]

Thanks for the reminder on this, dasa. I just merged main into the PR source branch.

[dasa]

Looks like this might need to be updated too:

emscripten/test/third_party/libpng/pngtest.c

Line 2031 in e6ad2d1

typedef png_libpng_version_1_6_55 Your_png_h_is_not_version_1_6_55;

[dasa]

@HCL-JasonR maybe another merge from main will fix this?

[dasa]

Friendly ping 😄

[sbc100]

@HCL-JasonR it looks like we (emscripten maintainers) don't have permission to rebase your branch to fix the conflicts. Would you have time to do it?

If not we can ask @dasa to take this on in a separate PR?

[HCL-JasonR]

My apologies, this kept making my "fix it this afternoon" list and getting bumped.

I just merged emscripten-core/emscripten main with no merge conflicts and pushed the updates.