... when height and width are different

SVG specifies embedded graphics to keep their aspect ratio (thanks to Regina for pointing out).
To implement this for Office SVG import, I had to correctly set the graphic data unit dimensions to make the existing SvgAspectRatio mapping work.
All together this now simplifies this method and packs all in clean linear transformations in the created stack of drwainglayer primitives.

Change-Id: Id80c2f74ffb148085085b2c9627bc31bc15bdee5
Reviewed-on: https://gerrit.libreoffice.org/21830Reviewed-by: Armin Le Grand <Armin.Le.Grand@cib.de>
Tested-by: Armin Le Grand <Armin.Le.Grand@cib.de>