Optimise QgsFeaturePool construction

- Avoid double iteration over complete layer when all features are
being used
- Avoid iterating over non-required features when selected features
are being used

Author: nyalldawson

src/analysis/vector/geometry_checker/qgsfeaturepool.cpp

@@ -32,25 +32,23 @@ QgsFeaturePool::QgsFeaturePool( QgsVectorLayer *layer, double layerToMapUnits, c
   , mLayerToMapTransform( layerToMapTransform )
   , mSelectedOnly( selectedOnly )
 {
+  // Build spatial index
+  QgsFeature feature;
+  QgsFeatureRequest req;
+  req.setSubsetOfAttributes( QgsAttributeList() );
   if ( selectedOnly )
   {
     mFeatureIds = layer->selectedFeatureIds();
-  }
-  else
-  {
-    mFeatureIds = layer->allFeatureIds();
+    req.setFilterFids( mFeatureIds );
   }
 
-  // Build spatial index
-  QgsFeature feature;
-  QgsFeatureRequest req;
-  req.setSubsetOfAttributes( QgsAttributeList() );
   QgsFeatureIterator it = layer->getFeatures( req );
   while ( it.nextFeature( feature ) )
   {
-    if ( mFeatureIds.contains( feature.id() ) && feature.geometry() )
+    if ( feature.geometry() )
     {
       mIndex.insertFeature( feature );
+      mFeatureIds.insert( feature.id() );
     }
     else
     {