Fixed wrong rendering offset / clipping with shadowBlur in Canvas.

If the item being rendered didn't have a bounding rect starting in the
origin the shadow path would be get an additional offset equal to the
distance of the item from the origin. Also, the stroke's shadow did not
take the pen width into account, causing clipping artifacts.

Combine all the common shadow generating code into a single function,
and avoid rendering into an intermediate image that is then blitted into
a larger image.

Task-number: QTBUG-30914
Change-Id: I16c7c15897ab8e2d46f37da835f75f97fe680863
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>