(clarifying that the "lang" and "lang_key" fields can be NULL pointers
or might not exist).

on a variety of images, limited documentation (in the header file.)

the sBIT fields in the test pixel as 0, which resulted in a floating
point division by zero which was irrelevant but causes systems where
FP exceptions cause a crash. Added code to pngvalid to turn on FP
exceptions if the appropriate glibc support is there to ensure this is
tested in the future.

It is too risky. Exactly how png_ptr->zbuf is used is under control of the
individual chunk implementation and there could easily be significant changes
within a major release.