Build against & use libpng 1.5+ #2
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…n libjasper. Likely fixed some longstanding bugs with PNG comment handling, which wasn't correct in the old code but was on a rarely-triggered code path. This version of xv compiles & runs on Debian Bullseye and other modern Linux/Unixes.
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Libpng 1.4 deprecated and libpng 1.5 finally abolished the direct access to the png_info structure that xv was using. There are a couple of other forks on GH that claim to build and work against modern libpng but some of the diffs looked a little shaky to me so I thought I'd start from scratch.
I did make some changes to the PNG comment handling, which appears to have been bugged in legacy xv (the tp pointer was not properly initialized in WritePNG if max_text was already > num_text, and the pointer was not properly advanced when adding non-keyword comments). Other than that the changes are solely what's required by the libpng changes, although it's tempting to start a wider cleanup effort. I also commented out the jasper (JPEG 2000 library) dependency in the Makefile, which lets this build on modern distributions where libjasper is not always easily available.
Built & tested against libpng 1.6.37 on Debian Bullseye (11.4), for both reading and writing.