Avoid some unnecessary styleChanged signals

These signals are expensive to emit, because they trigger a full
update of the style dock contents

src/core/qgsmaplayer.cpp

@@ -207,8 +207,11 @@ QgsRectangle QgsMapLayer::extent() const
   return mExtent;
 }
 
-void QgsMapLayer::setBlendMode( QPainter::CompositionMode blendMode )
+void QgsMapLayer::setBlendMode( const QPainter::CompositionMode blendMode )
 {
+  if ( mBlendMode == blendMode )
+    return;
+
   mBlendMode = blendMode;
   emit blendModeChanged( blendMode );
   emit styleChanged();

src/core/qgsvectorlayer.cpp

@@ -3942,6 +3942,9 @@ QVariant QgsVectorLayer::aggregate( QgsAggregateCalculator::Aggregate aggregate,
 
 void QgsVectorLayer::setFeatureBlendMode( QPainter::CompositionMode featureBlendMode )
 {
+  if ( mFeatureBlendMode == featureBlendMode )
+    return;
+
   mFeatureBlendMode = featureBlendMode;
   emit featureBlendModeChanged( featureBlendMode );
   emit styleChanged();
@@ -3954,6 +3957,8 @@ QPainter::CompositionMode QgsVectorLayer::featureBlendMode() const
 
 void QgsVectorLayer::setOpacity( double opacity )
 {
+  if ( qgsDoubleNear( mLayerOpacity, opacity ) )
+    return;
   mLayerOpacity = opacity;
   emit opacityChanged( opacity );
   emit styleChanged();