Port away from QRegion::rects()

Use begin()/end() instead.

Drop the special handling of rectCount() == 1, since with begin()/end(),
we no longer take the hit of creating a QVector just to be able to return
it from rects().

Change-Id: I4c03e10f7a8bfea87cb04aed7b87410991020a89
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>