'Return a |Png| instance having header properties parsed from image in
*stream*.'

@classmethod
def from_stream(cls, stream):

    parser = _PngParser.parse(stream)
    px_width = parser.px_width
    px_height = parser.px_height
    horz_dpi = parser.horz_dpi
    vert_dpi = parser.vert_dpi
    return cls(px_width, px_height, horz_dpi, vert_dpi)
