PNG nonconformance: duplicate eXIf chunks.

[jbowler]

I'm seeing files produced in November 2024 and later and apparently originally authored by PhotoShop 2024 and/or LightRoom which have the same eXIf chunk duplicated both before and after IDAT. This may have been fixed by the change in July 2023 which, so far as I can determine, prevents are write after IDAT if you occurred before and, anyway, I didn't think PhotoShop used libpng.

This issue is a placeholder in case anyone else sees it. libpng appears to be correct and it looks like the bug was fixed in July 2023

[jbowler]

#645 provides the mechanism to reliably prevent this happening with "one only" chunks which may occur both before and after IDAT. eXIf was the only example (other chunks may occur multiple times).

#645 does not contain the write side of the solution but it will work on read; only the first eXIf will be accepted regardless of the app playing mix'n'match with png_info. Still a WiP.