Set the leftmost position correctly based on logical advancement
values of XTextLayout to render the substring of a RTL text.

Change-Id: Ib65b433105274f6ebfd251e92d831f1b28516294
Reviewed-on: https://gerrit.libreoffice.org/61459
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>