exporter: start debug timer before EncodeFrame() call

Minor code path improvement. Since EncodeFrame() will try to stop the debug
timer if the encode is finished, it doesn't make sense to always start it
unconditionally after calling EncodeFrame().

app/render/backend/exporter.cpp

@@ -196,9 +196,9 @@ void Exporter::FrameRendered(const rational &time, FramePtr value)
 
   qDebug() << "    Waiting for" << waiting_for_frame_.toDouble();
 
-  EncodeFrame();
-
   debug_timer_.start();
+
+  EncodeFrame();
 }
 
 void Exporter::AudioRendered()