Minor speed cleanup

This adjusts iterators to use const reference when only used for
copy.  It also ensures pre-allocation of vectors when size is known
ahead of time.

common/plotters/PS_plotter.cpp

@@ -206,6 +206,7 @@ void PSLIKE_PLOTTER::FlashPadRoundRect( const wxPoint& aPadPos, const wxSize& aS
     std::vector< wxPoint > cornerList;
     // TransformRoundRectToPolygon creates only one convex polygon
     SHAPE_LINE_CHAIN& poly = outline.Outline( 0 );
+    cornerList.reserve( poly.PointCount() );
 
     for( int ii = 0; ii < poly.PointCount(); ++ii )
         cornerList.emplace_back( poly.Point( ii ).x, poly.Point( ii ).y );