SerialConnect: fix font metrics for good.

* Set the background color in AttachedToWindow to avoid white lines in
initial drawing,
* Fix computation of font size again so lines don't overlap.

Note: lines are apparently spaced 1px less than in Terminal. But they
don't seem to touch or overlap each other in SerialConnect.
Import libvterm r617.
libvterm: C89 backport.

* As a separate commit so next time the update will be easier...