Word appears to use negative z-indexes to indicate behind-text
zordering and positive numbers for in-front placement. This
was added for shapes in LO4.3, now applying to textboxes also.

Change-Id: I3b06fb231329f151ca978f3a68b4d4e89bc28515
Reviewed-on: https://gerrit.libreoffice.org/39671Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>