Fixed incorrect sample counter reported when rendering several render layers

intern/cycles/render/session.cpp

@@ -532,6 +532,9 @@ void Session::run()
 
 	/* run */
 	if(!progress.get_cancel()) {
+		/* reset number of rendered samples */
+		progress.reset_sample();
+
 		if(device_use_gl)
 			run_gpu();
 		else

intern/cycles/util/util_progress.h

@@ -120,6 +120,13 @@ public:
 		tile_time_ = tile_time;
 	}
 
+	void reset_sample()
+	{
+		thread_scoped_lock lock(progress_mutex);
+
+		sample = 0;
+	}
+
 	void increment_sample()
 	{
 		thread_scoped_lock lock(progress_mutex);