Fix cannot set line symbol data defined properties for vector

field marker (refs #15131)

Author: nyalldawson

src/core/symbology-ng/qgsvectorfieldsymbollayer.cpp

@@ -283,7 +283,7 @@ void QgsVectorFieldSymbolLayer::drawPreviewIcon( QgsSymbolV2RenderContext& conte
 
 QSet<QString> QgsVectorFieldSymbolLayer::usedAttributes() const
 {
-  QSet<QString> attributes;
+  QSet<QString> attributes = QgsMarkerSymbolLayerV2::usedAttributes();
   if ( !mXAttribute.isEmpty() )
   {
     attributes.insert( mXAttribute );
@@ -292,6 +292,10 @@ QSet<QString> QgsVectorFieldSymbolLayer::usedAttributes() const
   {
     attributes.insert( mYAttribute );
   }
+  if ( mLineSymbol )
+  {
+    attributes.unite( mLineSymbol->usedAttributes() );
+  }
   return attributes;
 }