code: def from_offset(cls, chunk_type, stream_rdr, offset):
          
          px_width = stream_rdr.read_long(offset)
          px_height = stream_rdr.read_long(offset, 4)
          return cls(chunk_type, px_width, px_height)
docs: Return an _IHDRChunk instance containing the image dimensions
          extracted from the IHDR chunk in *stream* at *offset*.