SLA support points (better backend->frontend synchronization and more)

- backend to frontend data synchronization to inform the gizmo that new points have been generated
- fixed the inadvertent cancellation of background processing caused by the SLA gizmo touching frontend data during the process
- inactive instances of the object are hidden when the SLA gizmo is active
- fix of imgui combobox rendering

src/slic3r/GUI/Plater.cpp

@@ -2231,6 +2231,7 @@ void Plater::priv::on_slicing_update(SlicingStatusEvent &evt)
     }
     if (evt.status.flags & PrintBase::SlicingStatus::RELOAD_SLA_SUPPORT_POINTS) {
         // Update SLA gizmo
+        q->canvas3D()->reload_scene(true);
     }
 }