Image stretch for maxwidth/maxheight

[pgundlach]

The image should grow if the "natural width" or "natural height" of an image is smaller than maxwidth or max height:

<Layout xmlns="urn:speedata.de:2009/publisher/en"
    xmlns:sd="urn:speedata:2009/publisher/functions/en">
    <Trace grid="yes"/>
    <Record element="data">
        <PlaceObject>
            <Image file="cow.pdf" maxwidth="15cm" maxheight="15cm"></Image>
        </PlaceObject>
    </Record>
</Layout>

The file cow.pdf is approx. 10cm x 7cm, it should (with a switch) grow to 15 * 11 or so.