Handle the case when the horizontal center is at 50%. Other cases are
still unhandled, those are more complex.

After fixing the style, center and border, the gradient fill looks
similar to how PowerPoint renders it.

Change-Id: I419da70482de37031aa2c7fc735692019d7665f5
Reviewed-on: https://gerrit.libreoffice.org/67175Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>