SDLPP: make copySelf actually copy everything

sdlpp/sdlpp_circlecolider.cpp

@@ -130,4 +130,8 @@ void CircleColider::render( Renderer &renderer ) {
 int CircleColider::getRadius() const {
     return rad_;
 }
+
+std::shared_ptr< CollisionPolygon > CircleColider::copySelf() {
+    return std::make_shared< CircleColider >( *this );
+}
 } // namespace SDLPP