[libpng16] Added calls to png_handle_eXIf(() in pngread.c and png_write_eXIf()

  in pngwrite.c, and made various other fixes to png_write_eXIf().
Eliminated png_ptr->num_exif member from pngstruct.h and added num_exif
  to arguments for png_get_eXIf() and png_set_eXIf().
8 files changed