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supplied although it doesn't have to do anything. If
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PNG_WRITE_FLUSH_SUPPORTED is not defined at libpng compile
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time, output_flush_fn will be ignored, although it must be
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png_set_write_fn(png_structp png_ptr, png_voidp io_ptr,
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png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn)