fix drawing box chars, avoid storing and saving state all the time

Summary:
to get the box chars to be drawn correctly we need to turn on high
quality antialiasing in qpainter. in addition only turn it on if
antialiasing is enabled.

lastly qpainter.save()/restore() is called very often, so try to avoid
that if it isn't necessary.

BUG: 401463

Test Plan:
cat tests/boxes.txt

old:

new:

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: wbauer, konsole-devel, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D16947