Hollowing gizmo can now actually trigger the hollowing and render result

src/slic3r/GUI/GLCanvas3D.hpp

@@ -486,6 +486,7 @@ public:
     void set_color_by(const std::string& value);
 
     const Camera& get_camera() const { return m_camera; }
+    const Shader& get_shader() const { return m_shader; }
 
     BoundingBoxf3 volumes_bounding_box() const;
     BoundingBoxf3 scene_bounding_box() const;