svp clipping again: Rectangle dimensions don't map directly to B2IBox.

This is the same as in 80f3211a7aeff221718703c445be4c753e0a1067.

Change-Id: Ibb981da064b4fc97fd865be2decabd8798de7380
Reviewed-on: https://gerrit.libreoffice.org/10186
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>