Log Message:
Changes 1.7.56:
  Only use pngcrush_debug_malloc() and pngcrush_debug_free() if the result
    is going to be shown.
  Added PNG_PASS_ROWS, PNG_UNUSED, and other macro definitions, when building
    with libpng-1.4.x and older libpng versions.
  Multiplied rowbytes by 8/bit_depth when using the system library because
    we do not call png_read_transform_info(). This prevents a crash when
    reading sub-8-bit input files.