Improve graphical console performance

Improve graphical console performance

As it is pretty pointless to clear unused parts of a line over and
over again, keep track of how much of a line was actually written
to and avoid clearing parts of the screen that are known to already
be clear. With this, scrolling speed becomes comparable to that of
Linux' VESA console.

Signed-off-by: Jan Beulich <jbeulich@novell.com>