screenshot: Composite multiple captures into one image

When clutter gives us multiple captures (multiple cairo_surface_t's),
composite them into one large image and use that as final screenshot
result. This makes screenshooting work when mutter uses multiple views.

https://bugzilla.gnome.org/show_bug.cgi?id=770128

src/shell-util.h

@@ -51,6 +51,13 @@ gboolean shell_util_need_background_refresh (void);
 ClutterContent * shell_util_get_content_for_window_actor (MetaWindowActor *window_actor,
                                                           MetaRectangle   *window_rect);
 
+cairo_surface_t * shell_util_composite_capture_images (ClutterCapture  *captures,
+                                                       int              n_captures,
+                                                       int              x,
+                                                       int              y,
+                                                       int              width,
+                                                       int              height);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */