Bringing back multithreaded thumbnails creation ; was commented out in changeset 1be6f248cb68 on 2013-01-10, see issue 1575

rtgui/thumbimageupdater.cc

@@ -66,10 +66,9 @@ public:
 		inactive_waiting_(false)
 	{
 		int threadCount=1;
-		/* Test: Only 1 thread for thumbnails creation, since tools are already parallelized
 		#ifdef _OPENMP
 		threadCount=omp_get_num_procs();  
-		#endif */
+		#endif
 		
 		threadPool_=new Glib::ThreadPool(threadCount,0);
 	}