Don't stroke a path if it's not specified in the SVG

The code in the svg renderer is using a QPen with it's brush
set to Qt::NoBrush as a hack to indicate that we don't have a
stroke to paint, but to nevertheless use QPen to keep track of
state. So when rendering, we have to check for the pen having
no brush and in that case not draw anything.

Task-number: QTBUG-36767
Change-Id: Ib622c7d0108e7360437f57853f11c65190c16773
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Correctly handle Miter joins in all cases

SVG uses SvgMiterJoin, not the Qt definition of miter
joins at all times.

Change-Id: I85b8e13609fa37384f09457f724a42d720769028
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>