Fix pads python api compatibility

* Add GetCustomShapeAsPolygon() to swig wrapper
* Make shared_ptr<SHAPE_POLY_SET> usable for GetEffectivePolygon()

Fixes #4999

common/swig/kicad.i

@@ -135,6 +135,10 @@ typedef long time_t;
 // KiCad plugin handling
 %include "kicadplugins.i"
 
+%shared_ptr(SHAPE)
+%shared_ptr(SHAPE_POLY_SET)
+%shared_ptr(SHAPE_LINE_CHAIN)
+
 #include <geometry/shape.h>
 %include <geometry/shape.h>
 

pcbnew/swig/pad.i

@@ -41,5 +41,11 @@
             return self.AddPrimitiveCurve(*args)
         else:
             raise TypeError("Arguments not recognized.")
+
+    # GetCustomShapeAsPolygon() is the old accessor to get custom shapes
+    def GetCustomShapeAsPolygon(self):
+        polygon_set = SHAPE_POLY_SET()
+        self.MergePrimitivesAsPolygon(polygon_set)
+        return polygon_set
     %}
 }