api: Deprecate legacy pointer typedefs
fix: Use `png_voidp` instead of `voidp` in pngread.c and pngwrite.c
doc: Move version numbering history from png.h to libpng-history.txt
refactor: Tidy up local definitions in pngerror.c
refactor: Tidy up local definitions in pngerror.c
doc: Update AUTHORS.md
Simplify png_formatted_warning
Help compilers optimize pngerror.o size