libweston: Add view unmap listener to pointer constraints

Since the logic of pointer constraints assumes a valid view throughout, add a
signal to disable constraints when its current view is unmapped by Weston.

The assumption that a previously unmapped view is valid already leads to the
constraints code crashing. This can happen when attaching a NULL buffer to the
surface and commiting, which effectively unmaps the view with the side effect of
clearing the surface's input region, which is then assumed valid inside
maybe_warp_confined_pointer().

Fixes: #721

Signed-off-by: Sergio Gómez <sergio.g.delreal@gmail.com>
(cherry picked from commit e3079393c400e3dc6498234d1d092f3072fa8b44)

include/libweston/libweston.h

@@ -1452,6 +1452,7 @@ struct weston_view {
 	struct weston_surface *surface;
 	struct wl_list surface_link;
 	struct wl_signal destroy_signal;
+	struct wl_signal unmap_signal;
 
 	/* struct weston_paint_node::view_link */
 	struct wl_list paint_node_list;
@@ -1605,6 +1606,7 @@ struct weston_pointer_constraint {
 	bool hint_is_pending;
 
 	struct wl_listener pointer_destroy_listener;
+	struct wl_listener view_unmap_listener;
 	struct wl_listener surface_commit_listener;
 	struct wl_listener surface_activate_listener;
 };