wayland: Do not reset frame list when merging pending state

In the synchronized subsurface case, the destination list may
contain other elements from previous wl_surface.commit calls.
Resetting the list will leave those dangling frame callbacks
that will lead to invalid writes when those get to be destroyed
(eg. on client shutdown).

src/wayland/meta-wayland-surface.c

@@ -468,7 +468,6 @@ merge_pending_state (MetaWaylandPendingState *from,
       to->dy = from->dy;
     }
 
-  wl_list_init (&to->frame_callback_list);
   wl_list_insert_list (&to->frame_callback_list, &from->frame_callback_list);
 
   cairo_region_union (to->surface_damage, from->surface_damage);