implemented volume and pan effects

This commit implements a structure for sample processing and two implemenations
of this structure in volume and pan effects.

app/render/backend/exporter.cpp

@@ -145,7 +145,7 @@ void Exporter::FrameRendered(const rational &time, QVariant value)
     } while (cached_frames_.contains(waiting_for_frame_));
 
     if (waiting_for_frame_ >= viewer_node_->Length()) {
-S      ExportSucceeded();
+      ExportSucceeded();
     }
   } else {
     cached_frames_.insert(time, value);