Smooth scale cover art for notifications, and display an empty album cover for tracks that have no artwork

data/data.qrc

@@ -61,5 +61,6 @@
         <file>open_stream.png</file>
         <file>schema-1.sql</file>
         <file>schema-2.sql</file>
+        <file>nocover.png</file>
     </qresource>
 </RCC>

src/osd_x11.cpp

@@ -66,7 +66,8 @@ void OSD::ShowMessageNative(const QString& summary, const QString& message,
 void OSD::ShowMessageNative(const QString& summary, const QString& message,
                             const QImage& image) {
 #ifdef HAVE_LIBNOTIFY
-  QImage happy_gdk_image = image.convertToFormat(QImage::Format_RGB888).scaledToHeight(100);
+  QImage happy_gdk_image = image.scaledToHeight(100, Qt::SmoothTransformation)
+                           .convertToFormat(QImage::Format_RGB888);
   pixbuf_ = gdk_pixbuf_new_from_data(
       happy_gdk_image.bits(),
       GDK_COLORSPACE_RGB,

src/song.cpp

@@ -378,5 +378,5 @@ QImage Song::GetBestImage() const {
   if (!d->art_automatic_.isEmpty())
     return QImage(d->art_automatic_);
 
-  return QImage();
+  return QImage(":/nocover.png");
 }