window-actor: Add API to get a cairo surface of the window

This currently uses a hack where it pushes a CoglFramebuffer backed by a
texture to the framebuffer stack, then calls clutter_actor_paint() on
the window actor causing it to render into the framebuffer. This has the
effect that all subsurfaces of a window will be drawn as part of the
window.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/752

src/meta/meta-window-actor.h

@@ -47,6 +47,10 @@ void               meta_window_actor_sync_visibility      (MetaWindowActor *self
 META_EXPORT
 gboolean       meta_window_actor_is_destroyed (MetaWindowActor *self);
 
+META_EXPORT
+cairo_surface_t * meta_window_actor_get_image (MetaWindowActor       *self,
+                                               cairo_rectangle_int_t *clip);
+
 typedef enum
 {
   META_SHADOW_MODE_AUTO,