Antialias line drawing characters

Summary:
BUG: 397985

Use antialiasing when drawing unicode box characters to improve the
rendering of rounded corners.

Test Plan:

1. Execute echo -e '\u256d\u256e\n\u2570\u256f'
2. The box should have rounded corners

Before fix:

After fix:

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: ngraham, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D15915