background: Mipmap wallpaper when shrinking

So as to minimize jaggies for wallpaper that is >= double the resolution
of the screen.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/254

src/compositor/meta-background.c

@@ -629,6 +629,11 @@ create_pipeline (PipelineType type)
       cogl_pipeline_set_blend (templates[type], blend_strings[type], NULL);
     }
 
+  cogl_pipeline_set_layer_filters (templates[type],
+                                   0,
+                                   COGL_PIPELINE_FILTER_LINEAR_MIPMAP_NEAREST,
+                                   COGL_PIPELINE_FILTER_LINEAR);
+
   return cogl_pipeline_copy (templates[type]);
 }