[ ] benchmark accessing very last PNG scanline
[ ] use iostruct

ways to hide info in PNG:
 * IHDR: longer than 13 bytes
 * IEND:
    * longer than  0 bytes
    * data after IEND
 * TEXT chunks
 * zTXT chunks
    * comp_method
    * data after compressed data?
 * IDAT:
    * data after last scanline
    * last bits in scanline when bpp%8 != 0
 * PLTE:
    * raw letters
    * stegano
    * many palette entries with same color => visually same pixels but, different color idx
 * custom chunks
 * crc
