libweston: Add paint node destruction into weston_layer_entry_remove

This prevents a potential crash where users of weston_layer_entry_insert/layer_entry_remove() would see when moving views into a NULL layer (effectively unmapping the surface/view).

Users that have migrated to the weston_view_move_to_layer() are immune to this issue because that takes care paint node destruction.

Alternatively another fix would have been to add the paint node destruction into weston_layer_entry_remove().

Signed-off-by: Marius Vlad marius.vlad@collabora.com