fdo#77454: fix WW8 import/export of negative image crop

The negative crop values were imported as large positive values, which
caused the image to be rendered with 1 pixel width after commit
2e5167528f7566dd9b000e50fc1610b7bf99132a.

(cherry picked from commit 6d431ffb682d0e64b75b6267f369822ff0b0617e)

Conflicts:
	sw/qa/extras/ww8export/ww8export.cxx

Change-Id: I0e01b9d9a05d90e868699832085a06ba5aab7e54
Reviewed-on: https://gerrit.libreoffice.org/9470
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>