Don't label preview tiles

Labeling can be expensive, so don't do this for tiles which
are only going to be used as a preview image

Author: nyalldawson

src/gui/qgsmapcanvas.cpp

@@ -2154,6 +2154,7 @@ void QgsMapCanvas::startPreviewJobs()
       jobExtent.setYMinimum( jobExtent.yMinimum() + dy );
 
       jobSettings.setExtent( jobExtent );
+      jobSettings.setFlag( QgsMapSettings::DrawLabeling, false );
 
       QgsMapRendererQImageJob *job = new QgsMapRendererParallelJob( jobSettings );
       mPreviewJobs.append( job );