Make modifications through the style dock much faster

Before any change in the style dock would invalidate the cache
for ALL map layers, resulting in every layer being redrawn.

Now only the affected layer is redrawn and all others just use
the cached render.

(cherry-picked from c6436f8)

src/app/qgslayerstylingwidget.cpp

@@ -267,8 +267,7 @@ void QgsLayerStylingWidget::apply()
   {
     emit styleChanged( mCurrentLayer );
     QgsProject::instance()->setDirty( true );
-    mMapCanvas->clearCache();
-    mMapCanvas->refresh();
+    mCurrentLayer->triggerRepaint();
   }
   connect( mCurrentLayer, SIGNAL( styleChanged() ), this, SLOT( updateCurrentWidgetLayer() ) );
 }