also update usedAttributes with attributes used in SVG parameters

python/core/auto_generated/symbology/qgsmarkersymbollayer.sip.in

@@ -708,6 +708,9 @@ Returns the units for the stroke width.
     virtual void prepareExpressions( const QgsSymbolRenderContext &context );
 
 
+    virtual QSet<QString> usedAttributes( const QgsRenderContext &context ) const;
+
+
   protected:
 
     double calculateAspectRatio( QgsSymbolRenderContext &context, double scaledSize, bool &hasDataDefinedAspectRatio ) const;

src/core/symbology/qgsmarkersymbollayer.cpp

@@ -3548,3 +3548,15 @@ void QgsSvgMarkerSymbolLayer::prepareExpressions( const QgsSymbolRenderContext &
 
   QgsMarkerSymbolLayer::prepareExpressions( context );
 }
+
+
+QSet<QString> QgsSvgMarkerSymbolLayer::usedAttributes( const QgsRenderContext &context ) const
+{
+  QSet<QString> attrs = QgsMarkerSymbolLayer::usedAttributes( context );
+
+  QMap<QString, QgsProperty>::const_iterator it = mParameters.constBegin();
+  for ( ; it != mParameters.constEnd(); ++it )
+    attrs.unite( it.value().referencedFields( context.expressionContext() ) );
+
+  return attrs;
+}

src/core/symbology/qgsmarkersymbollayer.h

@@ -629,6 +629,8 @@ class CORE_EXPORT QgsSvgMarkerSymbolLayer : public QgsMarkerSymbolLayer
 
     void prepareExpressions( const QgsSymbolRenderContext &context ) override;
 
+    QSet<QString> usedAttributes( const QgsRenderContext &context ) const override;
+
   protected:
 
     /**