QPolygonF doesn't have test coverage

There is no test coverage for QPolygonF at all, and only limited for QPolygon. Instead mostly it's relied that QPainterPath does the right thing, which it really doesn't for anything but the most trivial case - treating the polygon as a list of points.

A very simple examples are ::intersected and ::united not being symmetric:

QPolygon a(...), b(...);
Q_ASSERT(a.intersected(b) == b.intersected(a));

fails.