Previously, bidi portions did not handle underflow in word wrapping.
This caused bidi portions to break at format changes during wrapping,
instead of backtracking to a valid break opportunity.
Change-Id: Ib86effe2642a9d133242c28538026d9b4a7e90e9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/171006
Tested-by: Jenkins
Reviewed-by: Jonathan Clark <jonathan@libreoffice.org>
