wrong (high by one) 25% of the time. Dividing by 257 with rounding is
    wrong in 128 out of 65536 cases. Getting the right answer all the time
    without division is easy.

only be used to reduce the defined limit, and that it also affects
png writing.

  libpng-manual.txt.
  The cHRM chunk now sets the defaults for png_set_rgb_to_gray() (when negative
    parameters are supplied by the caller), while in the absence of cHRM
    sRGB/Rec 709 values are still used.
  The bKGD chunk no longer overwrites the background value set by
    png_set_background(), allowing the latter to be used before the file
    header is read. It never performed any useful function to override
    the default anyway.

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PNG_AFTER_IDAT from pngpriv.h to png.h because they must be visible to
applications that call png_set_unknown_chunks().

structure in the libpng documentation.