Draw empty arrowheads closed

Summary:
Because `drawPolyline` will not implicitely close the supplied Polygon
(as `drawPolygon` does), there was always the last edge of the arrowhead missing.
We have to manually close the Polygon by setting the endpoint to be the
starting point.

Test Plan:
Before:
{F7204345}
After:
{F7204349}

Reviewers: vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D23093