Hi,
I used to be a libpng developer, long ago. Although I have been much less active recently, I am still reading their mailing lists, and I can give you some (hopefully useful) hints.
I see that your change is applied to libpng-1.2. That branch is officially closed, they accept critical bug fixes only. The current branch is version 1.4, and version 1.5 is in late beta.
Their terminology is the following: version 1.2 is "stable", version 1.4 is "master", and version 1.5 is "devel".
There is a history with ASM code, there used to be a MMX-optimized version of png_read_filter_row that was eventually removed, partly because it wasn't that much better than the generic C code, and partly because it caused all kinds of configuration issues. I am saying this to make you aware that you _might_ encounter opposition with this submission.
The submission goes by posting an email to
png-mng-...@lists.sourceforge.net, and the people over there discuss the patch. There isn't one particular designated reviewer per submission, and there aren't people with commit rights. If there is consensus that the patch is okay, the maintainer, Glenn Randers-Pehrson, will commit the patch to their git repository.
Feel free to ask me for help. I can assist you while you're submitting this to png-mng-implement, or I can submit it myself, whichever you prefer.
Best regards,
Cosmin
♫