[pal] reverse iterate through layers

We want the highest layers in the layer tree to be handled first

src/core/pal/pal.cpp

@@ -163,13 +163,13 @@ std::unique_ptr<Problem> Pal::extractProblem( const QgsRectangle &extent, const
     candidateProfile = std::make_unique< QgsScopedRuntimeProfile >( QObject::tr( "Generating label candidates" ), QStringLiteral( "rendering" ) );
   }
 
-  for ( const auto &it : mLayers )
+  for ( auto it = mLayers.rbegin(); it != mLayers.rend(); ++it )
   {
     index++;
     if ( feedback )
       feedback->setProgress( index * step );
 
-    Layer *layer = it.second.get();
+    Layer *layer = it->second.get();
     if ( !layer )
     {
       // invalid layer name
@@ -181,12 +181,12 @@ std::unique_ptr<Problem> Pal::extractProblem( const QgsRectangle &extent, const
       continue;
 
     if ( feedback )
-      feedback->emit candidateCreationAboutToBegin( it.first );
+      feedback->emit candidateCreationAboutToBegin( it->first );
 
     std::unique_ptr< QgsScopedRuntimeProfile > layerProfile;
     if ( context.flags() & Qgis::RenderContextFlag::RecordProfile )
     {
-      layerProfile = std::make_unique< QgsScopedRuntimeProfile >( it.first->providerId(), QStringLiteral( "rendering" ) );
+      layerProfile = std::make_unique< QgsScopedRuntimeProfile >( it->first->providerId(), QStringLiteral( "rendering" ) );
     }
 
     // check for connected features with the same label text and join them
@@ -338,7 +338,7 @@ std::unique_ptr<Problem> Pal::extractProblem( const QgsRectangle &extent, const
     previousObstacleCount = obstacleCount;
 
     if ( feedback )
-      feedback->emit candidateCreationFinished( it.first );
+      feedback->emit candidateCreationFinished( it->first );
   }
 
   candidateProfile.reset();