background: When updating actor content, evict old content from cache

Normally backgrounds get evicted from the cache when their actor is
destroyed.  If the actor changes content without destroying itself,
though, we should evict the old content from the cache, too.

https://bugzilla.gnome.org/show_bug.cgi?id=696157

js/ui/background.js

@@ -372,6 +372,7 @@ const Background = new Lang.Class({
         content.brightness = this._brightness;
         content.vignette_sharpness = this._vignetteSharpness;
 
+        this._cache.removeImageContent(content);
         this._images[index].content = content;
         this._watchCacheFile(filename);
     },