In #i60945# it was discovered that Unix's leading external font spacing
causes problems with the display of documents. Therefore, the reference
device implemented a workaround, which was to set the spacing to zero.
However, the reference device is a VirtualDevice, so it should really be
handled there, not in OutputDevice. I have added a new protected function
to OutputDevice, GetFontExtLead() that handles this.

Change-Id: I1b84ee7d9f7ae96841b441b52705e67c8115ae5c