renderer: when invalidating visible, kick the workers into emit mode

app/render/backend/renderbackend.cpp

@@ -259,6 +259,8 @@ void RenderBackend::CacheNext()
       SetWorkerBusyState(worker, true);
       cancel_dialog_->WorkerStarted();
 
+      WorkerAboutToStartEvent(worker);
+
       QMetaObject::invokeMethod(worker,
                                 "Render",
                                 Qt::QueuedConnection,
@@ -471,6 +473,11 @@ void RenderBackend::CacheIDChangedEvent(const QString &id)
   Q_UNUSED(id)
 }
 
+void RenderBackend::WorkerAboutToStartEvent(RenderWorker *worker)
+{
+  Q_UNUSED(worker)
+}
+
 void RenderBackend::InitWorkers()
 {
   for (int i=0;i<processors_.size();i++) {