Do not show sinking contours when a gizmo hides some objects

src/slic3r/GUI/GLCanvas3D.cpp

@@ -5107,6 +5107,7 @@ void GLCanvas3D::_render_objects()
         m_volumes.set_z_range(-FLT_MAX, FLT_MAX);
 
     m_volumes.set_clipping_plane(m_camera_clipping_plane.get_data());
+    m_volumes.set_show_sinking_contours(! m_gizmos.is_hiding_instances());
 
     GLShaderProgram* shader = wxGetApp().get_shader("gouraud");
     if (shader != nullptr) {