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>
(cherry picked from commit a12f6ba302e54c1570c54aa4c722f2dafbf794af)
Fixes: assert in QPainter::fillRect on very large paint devices

For devices taller than 32768, fillRect with linear gradient would
go below the cliprect.

Task-number: QTBUG-26766
Change-Id: I516ede1545909e9ce2de5812c61bb473f99f5759
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
(cherry picked from qtbase/db775741ed57efc16c697471f5b87689df7105a8)