backend/wayland: Add active pointer per host seat

Every host seat with pointer capability propagates events to one of
sub-pointer depending which output window we entered.
active_pointer tracks reference to sub-pointer on enter/leave events to
avoid lookup for it on every move events.

Fixes swaywm/wlroots#1499

include/backend/wayland.h

@@ -116,6 +116,7 @@ struct wlr_wl_seat {
 	struct wl_keyboard *keyboard;
 
 	struct wlr_wl_backend *backend;
+	struct wlr_wl_pointer *active_pointer;
 };
 
 struct wlr_wl_backend *get_wl_backend_from_backend(struct wlr_backend *backend);