Always set a clip on the painter in QGraphicsView.

This allows items to check painter->clipRegion() to find out what the
imposed clip is, in order to cull elements that don't need to be
painted. This is an alternative approach to getting the same
information from QStyleOptionGraphicsItem::exposedRect. It's better
because it's a pull operation, but it's slightly worse because it
doesn't include the complete system clip, and because QRegion has
integer resolution only (whereas QGraphicsItem's coordinate uses
qreal.

A better approach may be to access QPainter's combined clip region;
this option is open for future versions of Qt.

Original patch by Warwick (which is why he's on reviewed-by), but the
patch was modified to operate in device instead of logical coordinates.

Reviewed-by: jasplin
Reviewed-by: Warwick Allison

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