wayland/xdg-shell: Cache pending frame callbacks on popup reset

A popup can be reset, and when that happens, window and actor are
destroyed, and won't be created again unless it is reassigned the
popup role.

If a client queued frame callbacks when resetting a popup, the frame
callbacks would be left in the pending state, as they were not queued on
the actor, meaning we'd hit an assert about the frame callbacks not
being handled. Fix this by caching them on the MetaWaylandSurface, so
that they either are cleaned up on destruction, or queued on the actor
would the surface be re-assigned the popup role.

https://gitlab.gnome.org/GNOME/mutter/issues/240

src/wayland/meta-wayland-xdg-shell.c

@@ -961,6 +961,7 @@ meta_wayland_xdg_popup_commit (MetaWaylandSurfaceRole  *surface_role,
   if (!surface->buffer_ref.buffer && xdg_surface_priv->first_buffer_attached)
     {
       meta_wayland_xdg_surface_reset (xdg_surface);
+      meta_wayland_surface_cache_pending_frame_callbacks (surface, pending);
       return;
     }