Gerber plotter: plot round rectangle pads using a region with arcs.

Previously, the region was a usual polygon with arc approximated by segment.
Using a region with arcs is a better way because it allows CAM tools to
identify this region as a round-rect pad.

A side effect is a better shape and smaller files, however not really noticeable in fact.

pcbnew/plot_brditems_plotter.cpp

@@ -792,6 +792,12 @@ void BRDITEMS_PLOTTER::PlotDrawSegment( DRAWSEGMENT* aSeg )
     if( aSeg->GetLayer() == Edge_Cuts )
         gbr_metadata.SetApertureAttrib( GBR_APERTURE_METADATA::GBR_APERTURE_ATTRIB_EDGECUT );
 
+    if( IsCopperLayer( aSeg->GetLayer() ) )
+        // Graphic items (DRAWSEGMENT, TEXT) having no net have the NonConductor attribute
+        // Graphic items having a net have the Conductor attribute, but are not (yet?)
+        // supported in Pcbnew
+        gbr_metadata.SetApertureAttrib( GBR_APERTURE_METADATA::GBR_APERTURE_ATTRIB_NONCONDUCTOR );
+
     switch( aSeg->GetShape() )
     {
     case S_CIRCLE: