Speed up point layer rendering - don't calculate unused label obstacles

Cuts render time by ~60%. Fix #15752.

(cherry-picked from 5798a82)

src/core/qgsvectorlayerrenderer.cpp

@@ -333,7 +333,7 @@ void QgsVectorLayerRenderer::drawRendererV2( QgsFeatureIterator& fit )
           }
         }
         // new labeling engine
-        if ( mContext.labelingEngineV2() )
+        if ( mContext.labelingEngineV2() && ( mLabelProvider || mDiagramProvider ) )
         {
           QScopedPointer<QgsGeometry> obstacleGeometry;
           QgsSymbolV2List symbols = mRendererV2->originalSymbolsForFeature( fet, mContext );