Improve SHAPE_LINE_CHAIN arc plotting.

It still doesn't get it quite right as it restarts
the lineStyle pattern at each segment/arc transition.

include/plotters/plotter_gerber.h

@@ -60,9 +60,13 @@ public:
                               double aScale, bool aMirror ) override;
 
     // Basic plot primitives
+
+    // Note: do not use Rect() to plot rounded-corner rectangles.  Use PlotPolyAsRegion() instead.
     virtual void Rect( const VECTOR2I& p1, const VECTOR2I& p2, FILL_T fill, int width,
                        int aCornerRadius = 0 ) override;
+
     virtual void Circle( const VECTOR2I& pos, int diametre, FILL_T fill, int width ) override;
+
     virtual void Arc( const VECTOR2D& aCenter, const EDA_ANGLE& aStartAngle,
                       const EDA_ANGLE& aAngle, double aRadius, FILL_T aFill, int aWidth ) override;