Make DRAWSEGMENT::GetRectCorners return vector

... instead of modifying the argument.
This will make the method usable in python API and will not incur
permormance penalty because named return value optimization (NRVO)
is a thing since C++11.
But even if copy is not elided vector is moved instead of copied.
https://en.cppreference.com/w/cpp/language/copy_elision

pcbnew/plot_brditems_plotter.cpp

@@ -506,8 +506,7 @@ void BRDITEMS_PLOTTER::PlotFootprintGraphicItem( EDGE_MODULE* aEdge )
 
     case S_RECT:
     {
-        std::vector<wxPoint> pts;
-        aEdge->GetRectCorners( &pts );
+        std::vector<wxPoint> pts = aEdge->GetRectCorners();
 
         if( aEdge->GetWidth() > 0 )
         {
@@ -868,8 +867,7 @@ void BRDITEMS_PLOTTER::PlotDrawSegment( DRAWSEGMENT* aSeg )
 
     case S_RECT:
     {
-        std::vector<wxPoint> pts;
-        aSeg->GetRectCorners( &pts );
+        std::vector<wxPoint> pts = aSeg->GetRectCorners();
 
         if( aSeg->GetWidth() > 0 )
         {