<img src="REF.TO.IMAGE" ... /> become <draw:image xlink:href="../REF.TO.IMAGE" ... />
  so remove "../" added by OOo
* compile regex only once

reviewed by Kazuhiko



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31648 20353a03-c40f-0410-a6d1-a30d3c3de9de