windowManager: add animations for fullscreen and unfullscreen

We use the newly introduced feature from Mutter to hook up our own
fullscreen and unfullscreen animations.
To give the illusion of a transition as smooth as possible, we create a
snapshot of the current contents of the actor before its state is
changed, and crossfade between the two states while the size changes.

https://bugzilla.gnome.org/show_bug.cgi?id=707248

src/shell-util.h

@@ -8,6 +8,7 @@
 #include <libsoup/soup.h>
 #include <gdk-pixbuf/gdk-pixbuf.h>
 #include <meta/meta-cursor-tracker.h>
+#include <meta/meta-window-actor.h>
 
 G_BEGIN_DECLS
 
@@ -50,6 +51,9 @@ gboolean shell_util_need_background_refresh (void);
 void shell_util_text_insert_keyval (ClutterActor *actor,
                                     guint         keyval);
 
+ClutterContent * shell_util_get_content_for_window_actor (MetaWindowActor *window_actor,
+                                                          MetaRectangle   *window_rect);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */