tspan element only valid as child of text element
if exporting a selection use top left corner of bounding rect as top left corner of viewbox
export x coordinate of text position for text portions

Change-Id: I8b739085473aa4a48ff3bbbbe8c413c3cddbaebd
Reviewed-on: https://gerrit.libreoffice.org/14992Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>