[labeling] When a layer is set to "blocking" label mode, correctly

handle data defined "is obstacle" and "priority" settings

(cherry picked from commit b46f499)

src/core/qgsvectorlayerlabelprovider.cpp

@@ -134,28 +134,31 @@ bool QgsVectorLayerLabelProvider::prepare( const QgsRenderContext &context, QSet
 
   lyr.mCurFields = mFields;
 
-  if ( lyr.drawLabels )
+  if ( lyr.drawLabels || lyr.obstacle )
   {
-    // add field indices for label's text, from expression or field
-    if ( lyr.isExpression )
+    if ( lyr.drawLabels )
     {
-      // prepare expression for use in QgsPalLayerSettings::registerFeature()
-      QgsExpression *exp = lyr.getLabelExpression();
-      exp->prepare( &context.expressionContext() );
-      if ( exp->hasEvalError() )
+      // add field indices for label's text, from expression or field
+      if ( lyr.isExpression )
       {
-        QgsDebugMsgLevel( "Prepare error:" + exp->evalErrorString(), 4 );
+        // prepare expression for use in QgsPalLayerSettings::registerFeature()
+        QgsExpression *exp = lyr.getLabelExpression();
+        exp->prepare( &context.expressionContext() );
+        if ( exp->hasEvalError() )
+        {
+          QgsDebugMsgLevel( "Prepare error:" + exp->evalErrorString(), 4 );
+        }
+        Q_FOREACH ( const QString &name, exp->referencedColumns() )
+        {
+          QgsDebugMsgLevel( "REFERENCED COLUMN = " + name, 4 );
+          attributeNames.insert( name );
+        }
       }
-      Q_FOREACH ( const QString &name, exp->referencedColumns() )
+      else
       {
-        QgsDebugMsgLevel( "REFERENCED COLUMN = " + name, 4 );
-        attributeNames.insert( name );
+        attributeNames.insert( lyr.fieldName );
       }
     }
-    else
-    {
-      attributeNames.insert( lyr.fieldName );
-    }
 
     lyr.dataDefinedProperties().prepare( context.expressionContext() );
     // add field indices of data defined expression or field