RTF import: fix ordering of old-style dhght in case of equal values

Commit 1eaab77c718ffa254068ae6032862dfb5a03db67 (fdo#60722 import
RTF_SHPZ, 2013-03-06) changed how we handle z-order, in case two shapes
have the same value. Turns out for drawing-objects the order is the
opposite in this situation.

So fix this by adding a new mode, that keeps the original testcase happy
without breaking older documents.

Change-Id: Ib2d284cefc3c0dce40ac2e516ba260d6cd04ce43