SHAPE_LINE_CHAIN: Remove element access

This is the first step to allowing non-segments in the line chain.
External routines cannot be allowed to change the line chain without
going through the internal routines.  To accomplish this, we remove the
Vertex() and Point() access routines and only leave the const versions.
Transformations are given for both points as well as the chain itself.

common/plotters/GERBER_plotter.cpp

@@ -902,7 +902,7 @@ void GERBER_PLOTTER::FlashPadRoundRect( const wxPoint& aPadPos, const wxSize& aS
     cornerList.reserve( poly.PointCount() + 1 );
 
     for( int ii = 0; ii < poly.PointCount(); ++ii )
-        cornerList.emplace_back( poly.Point( ii ).x, poly.Point( ii ).y );
+        cornerList.emplace_back( poly.CPoint( ii ).x, poly.CPoint( ii ).y );
 
     // Close polygon
     cornerList.push_back( cornerList[0] );
@@ -942,7 +942,7 @@ void GERBER_PLOTTER::FlashPadCustom( const wxPoint& aPadPos, const wxSize& aSize
         cornerList.clear();
 
         for( int ii = 0; ii < poly.PointCount(); ++ii )
-            cornerList.emplace_back( poly.Point( ii ).x, poly.Point( ii ).y );
+            cornerList.emplace_back( poly.CPoint( ii ).x, poly.CPoint( ii ).y );
 
         // Close polygon
         cornerList.push_back( cornerList[0] );