qt4: use slower and better scaling on albumart-label

We could use QPixmapCache here too, but I don't think it speeds up it
any noticeable way, just makes code littlebit longer.
(cherry picked from commit 0ba54f498eafa5b8c081ff37ecd65d791c3718e9)
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>

modules/gui/qt4/components/interface_widgets.cpp

@@ -587,7 +587,8 @@ void CoverArtLabel::showArtUpdate( const QString& url )
     if( !url.isEmpty() && pix.load( url ) )
     {
         pix = pix.scaled( maximumWidth(), maximumHeight(),
-                          Qt::KeepAspectRatioByExpanding );
+                          Qt::KeepAspectRatioByExpanding,
+                          Qt::SmoothTransformation );
     }
     else
     {