Check for attached dev gui for dt_dev_process_preview2()

We must do that as for the other dt_dev_process_xx() variants.

Author: jenshannoschwalm

src/develop/develop.c

@@ -238,6 +238,7 @@ void dt_dev_process_preview(dt_develop_t *dev)
 
 void dt_dev_process_preview2(dt_develop_t *dev)
 {
+  if(!dev->gui_attached) return;
   const gboolean err = dt_control_add_job_res(dt_dev_process_preview2_job_create(dev), DT_CTL_WORKER_ZOOM_2);
   if(err) dt_print(DT_DEBUG_ALWAYS, "[dev_process_preview2] job queue exceeded!");
 }