It interfered with nroff processing.

and update scripts/pnglibconf.h.prebuilt

being applied.

pngvalid still fails.

pngvalid still fails two tests, problem with row buffer.

to demonstrate chop_16_to_8

If this is not enabled, png_set_strip_16() and png_do_scale_16_to_8()
aren't built.

png_set_scale_16_to_8 from pngpriv.h

and fix some other small typos.