sw: fix fallback for OLE object output.

OLE objects could be written as graphics as a fallback for
unsupported formats, but it happened in a wrong position of the
document (inside w:rPr).

Added two extra fields to the PostponedGraphic structure to be able
to postpone the writing of OLE graphics.

Change-Id: If36a0aed11bcac2115ef0322cc83efccd6ef1204