In Word, the layer that contains a background image is behind the layer
that contains the paragraph background. In Writer, the paragraph
background is painted before the hell layer. Add a compat flag to change
the order, so the DOCX importer can trigger that.

To reproduce, create an XShape, send it to the background, set some
color for a paragraph background, and notice that the background color
is missing where the shape is behind the text.

Change-Id: I9b1fffd9ac9a6e5a1c3d1f65371440047d125b38