Line joins not painted correctly on the Mac

When drawing lines on the Mac which has a pen of the following type:

painter.setPen( QPen( QColor(255,0,0), 50, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin ) );

then the line joins are not drawn correctly. It works fine on Windows. This is probably an issue with Quartz2D, but we should investigate to see if we can work around this somehow.