Fixed dashes being rendered differently depending on system clip.

We need to clip lines to the unclipped device rect in the case of
dashing, since otherwise the dashes will be shifted and rendered
differently when partial repaints are done.

Task-number: QTBUG-24762
Change-Id: I3599b54baa552acc20bf8cc2e12f846b45f6019e
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>