SwAnchoredDrawObject::GetObjBoundRect: ignore relative size of group shapes

Instead of not reading the relative size of group shapes at all, read it
into the document model, just ignore it during painting. Visually this
doesn't change anything, however this way we can write the information
back to the file.

Change-Id: Ic24fceaacec0f831d657a35e52493e1c0e9bf626