background: free MetaBackground explicitly when its MetaBackgroundActor is destroyed

The memory which MetaBackground resides is freed automatically, when its
wrapper object Background is freed by garbage collection. But because the
memory for MetaBackground is huge, changing the background or changing the
background mode repeatedly makes a lot of memory to be in use until garbage
collection runs.

This patch frees the MetaBackground object explicitly when its
MetaBackgroundActor is destroyed which is the moment that the memory in the
object is not used anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=766353

Signed-off-by: Hyungwon Hwang <hyungwon.hwang7@gmail.com>

js/ui/background.js

@@ -696,6 +696,7 @@ const BackgroundManager = new Lang.Class({
                            time: FADE_ANIMATION_TIME,
                            transition: 'easeOutQuad',
                            onComplete: function() {
+                               oldBackgroundActor.background.run_dispose();
                                oldBackgroundActor.destroy();
                            }
                          });