libweston: add weston_layer_fini()

Layers did not have a fini sequence before, which means the compositor
layer list might have stale pointers temporarily when shutting down. A
bigger problem might be having views linger after the destruction of the
layer.

These problems were not observed yet, but if they exist, this patch
should help to find them and then fix them.

The check in weston_compositor_shutdown() is not an assert yet, because
it will trigger until all components call weston_layer_fini() correctly.
Some components do not even have a tear-down function to call it from at
all, like fullscreen-shell.

The same with the check in weston_layer_fini().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

include/libweston/libweston.h

@@ -1637,6 +1637,8 @@ void
 weston_layer_init(struct weston_layer *layer,
 		  struct weston_compositor *compositor);
 void
+weston_layer_fini(struct weston_layer *layer);
+void
 weston_layer_set_position(struct weston_layer *layer,
 			  enum weston_layer_position position);
 void