Invalid PAGE XML caused by PrintSpace with negative PageCoords

[stweil]

The NZZ PAGE XML file was created by Transkribus, and it contains data which is reported as invalid:

<Page imageFilename="0111_nzz_18901222_0_0_a1_p1_1.tif" imageWidth="3839" imageHeight="5551">
    <PrintSpace>
        <Coords points="4,-27 3842,-27 3842,5524 4,5524"/>
    </PrintSpace>

ocr-validate.py reports that negative values are invalid here.
PRImA page viewer refuses to load PAGE XML with such data.

See also issue #38 with other PAGE XML related problems.