Prevent redrawing cover image if size has not changed

Drawing the cover image on the Cover panel triggers the re-allocation of
the widget which in turn triggers the re-drawing of the cover image.
This led to an infinite loop of drawing the cover image causing high CPU
usage. To prevent this, the widget size is now stored and the the
resizing of the cover image is only done if the size has really changed.

mcg/widgets.py

@@ -810,6 +810,7 @@ class CoverPanel(GObject.GObject):
         self._tracklist_size = TracklistSize.LARGE
         self._icon_theme = Gtk.IconTheme.get_default()
         self._fullscreened = False
+        self._current_size = None
 
         # Widgets
         self._appwindow = builder.get_object('appwindow')
@@ -1066,8 +1067,15 @@ class CoverPanel(GObject.GObject):
         """Diese Methode skaliert das geladene Bild aus dem Pixelpuffer
         auf die Größe des Fensters unter Beibehalt der Seitenverhältnisse
         """
-        pixbuf = self._cover_pixbuf
+        # Get size
         size = self._cover_scroll.get_allocation()
+        # Abort if size is the same
+        if self._current_size and size.width == self._current_size.width and size.height == self._current_size.height:
+            return
+        self._current_size = size
+
+        # Get pixelbuffer
+        pixbuf = self._cover_pixbuf
         # Check pixelbuffer
         if pixbuf is None:
             return