Followup 7d600bd, proper fix for failing labeling engine test

When labelFeatures is called outside of the renderering loop,
we need to make sure the renderer is prepared by calling
startRender.

src/core/qgsvectorlayerlabelprovider.cpp

@@ -257,6 +257,9 @@ QList<QgsLabelFeature*> QgsVectorLayerLabelProvider::labelFeatures( QgsRenderCon
   if ( !prepare( ctx, attrNames ) )
     return QList<QgsLabelFeature*>();
 
+  if ( mRenderer )
+    mRenderer->startRender( ctx, mFields );
+
   QgsRectangle layerExtent = ctx.extent();
   if ( mSettings.ct )
     layerExtent = mSettings.ct->transformBoundingBox( ctx.extent(), QgsCoordinateTransform::ReverseTransform );
@@ -278,6 +281,9 @@ QList<QgsLabelFeature*> QgsVectorLayerLabelProvider::labelFeatures( QgsRenderCon
     registerFeature( fet, ctx, obstacleGeometry.data() );
   }
 
+  if ( mRenderer )
+    mRenderer->stopRender( ctx );
+
   return mLabels;
 }
 

src/core/symbology-ng/qgscategorizedsymbolrendererv2.cpp

@@ -243,8 +243,7 @@ QgsSymbolV2* QgsCategorizedSymbolRendererV2::originalSymbolForFeature( QgsFeatur
   QVariant value;
   if ( mAttrNum == -1 )
   {
-    if ( !mExpression.data() )
-      return 0;
+    Q_ASSERT( mExpression.data() );
 
     value = mExpression->evaluate( &context.expressionContext() );
   }

tests/src/core/testqgslabelingenginev2.cpp

@@ -142,7 +142,7 @@ void TestQgsLabelingEngineV2::testBasic()
 
   vl->setCustomProperty( "labeling/enabled", false );
 
-  QVERIFY( imageCheck( "labeling_basic_loop", img2, 0 ) );
+  QVERIFY( imageCheck( "labeling_basic", img2, 0 ) );
 }